Rstudio Server Authentication Callback URL

[systemnova]

When Rstudio calls:
token <- fitbitr::oauth_token()
with the callback URL being the server URL, it's opening in a new window (which closes the old session because Rstudio server detects the new window).
This new window doesnt capture the callback and so the oauth token is lost.
Any suggestions on how to best resolve?
If i change the callback URL to include https://HOST.com.au/__login__/callback
or https://HOST.com.au/callback
then RStudio Server returns "The requested page was not found"