diff --git a/__tests__/onInstallonOpen.test.ts b/__tests__/onInstallonOpen.test.ts
index c0782c8..4ef62a3 100644
--- a/__tests__/onInstallonOpen.test.ts
+++ b/__tests__/onInstallonOpen.test.ts
@@ -50,9 +50,7 @@ class MockUi {
     this.isAddonMenu = true;
     return this;
   }
-  addToUi(): void {
-    console.log(JSON.stringify(ADDON_MENU));
-  }
+  addToUi(): void {}
   createAddonMenu(): this {
     this.isAddonMenu = true;
     return this;
diff --git a/__tests__/verifyAuthKeyPrompt.test.ts b/__tests__/verifyAuthKeyPrompt.test.ts
new file mode 100644
index 0000000..dbbc441
--- /dev/null
+++ b/__tests__/verifyAuthKeyPrompt.test.ts
@@ -0,0 +1,100 @@
+import { verifyAuthKeyPrompt } from '../src/sheetsl';
+
+const verifyAuthKeyPromptSuccessPatterns = [
+  {
+    testName: 'Success',
+    input: {
+      promptResponse: {
+        getSelectedButton: () => 'ok',
+        getResponseText: () => 'ThisIsAnApiAuthKey:fx',
+      },
+      ui: {
+        Button: {
+          OK: 'ok', // Don't change this value.
+        },
+      },
+    },
+    expectedOutput: {
+      getSelectedButton: () => 'ok',
+      getResponseText: () => 'ThisIsAnApiAuthKey:fx',
+    },
+  },
+] as any[];
+
+const verifyAuthKeyPromptErrorPatterns = [
+  {
+    testName: 'Error: canceled auth key setting',
+    input: {
+      promptResponse: {
+        getSelectedButton: () => 'canceled',
+        getResponseText: () => 'ThisIsAnApiAuthKey:fx',
+      },
+      ui: {
+        Button: {
+          OK: 'ok', // Don't change this value.
+        },
+      },
+    },
+    expectedErrorMessage:
+      '[SheetsL] Canceled: Setting of DeepL Authentication Key has been canceled.',
+  },
+  {
+    testName: 'Error: empty auth key',
+    input: {
+      promptResponse: {
+        getSelectedButton: () => 'ok',
+        getResponseText: () => '',
+      },
+      ui: {
+        Button: {
+          OK: 'ok', // Don't change this value.
+        },
+      },
+    },
+    expectedErrorMessage:
+      '[SheetsL] You must enter a valid DeepL API Authentication Key.',
+  },
+  {
+    testName: 'Error: auth key is null',
+    input: {
+      promptResponse: {
+        getSelectedButton: () => 'ok',
+        getResponseText: () => null,
+      },
+      ui: {
+        Button: {
+          OK: 'ok', // Don't change this value.
+        },
+      },
+    },
+    expectedErrorMessage:
+      '[SheetsL] You must enter a valid DeepL API Authentication Key.',
+  },
+] as any[];
+
+describe.each(verifyAuthKeyPromptSuccessPatterns)(
+  'verifyAuthKeyPrompt Test: success patterns',
+  ({ testName, input, expectedOutput }) => {
+    test(`${testName}: getSelectedButton`, () => {
+      expect(
+        verifyAuthKeyPrompt(input.promptResponse, input.ui).getSelectedButton()
+      ).toBe(expectedOutput.getSelectedButton());
+    });
+    test(`${testName}: getResponseText`, () => {
+      expect(
+        verifyAuthKeyPrompt(input.promptResponse, input.ui).getResponseText()
+      ).toBe(expectedOutput.getResponseText());
+    });
+  }
+);
+
+describe.each(verifyAuthKeyPromptErrorPatterns)(
+  'verifyAuthKeyPrompt Test: error patterns',
+  ({ testName, input, expectedErrorMessage }) => {
+    test(testName, () => {
+      expect(() => {
+        verifyAuthKeyPrompt(input.promptResponse, input.ui);
+      }).toThrowError(new Error(expectedErrorMessage));
+    });
+  }
+);
diff --git a/src/sheetsl.ts b/src/sheetsl.ts
index 661db30..e6204aa 100644
--- a/src/sheetsl.ts
+++ b/src/sheetsl.ts
@@ -96,17 +96,7 @@ export function setDeeplAuthKey(): void {
       'Enter your DeepL API Authentication Key',
       ui.ButtonSet.OK_CANCEL
     );
-    if (promptResponse.getSelectedButton() !== ui.Button.OK) {
-      throw new Error(
-        `[${ADDON_NAME}] Canceled: Setting of DeepL Authentication Key has been canceled.`
-      );
-    }
-    const apiKey = promptResponse.getResponseText();
-    if (!apiKey || apiKey === '') {
-      throw new Error(
-        `[${ADDON_NAME}] You must enter a valid DeepL API Authentication Key.`
-      );
-    }
+    const apiKey = verifyAuthKeyPrompt(promptResponse, ui).getResponseText();
     PropertiesService.getUserProperties().setProperty(
       UP_KEY_DEEPL_API_KEY,
       apiKey
@@ -120,6 +110,32 @@ export function setDeeplAuthKey(): void {
   }
 }
 
+/**
+ * Verify the prompt response in setDeeplAuthKey and return an error
+ * if the prompt is canceled or if an invalid DeepL API Authentication Key
+ * was entered.
+ * @param promptResponse Response object for the user prompt in setDeeplAuthKey
+ * to enter the user's DeepL API Authentication Key.
+ * @returns The entered prompt response object.
+ */
+export function verifyAuthKeyPrompt(
+  promptResponse: GoogleAppsScript.Base.PromptResponse,
+  ui: GoogleAppsScript.Base.Ui
+): GoogleAppsScript.Base.PromptResponse {
+  if (promptResponse.getSelectedButton() !== ui.Button.OK) {
+    throw new Error(
+      `[${ADDON_NAME}] Canceled: Setting of DeepL Authentication Key has been canceled.`
+    );
+  }
+  const apiKey = promptResponse.getResponseText();
+  if (!apiKey || apiKey === '') {
+    throw new Error(
+      `[${ADDON_NAME}] You must enter a valid DeepL API Authentication Key.`
+    );
+  }
+  return promptResponse;
+}
+
 /**
  * Delete the stored DeepL API authentication key in user property.
  */