FormAssembly Redirect and stay inside VF

//most of this is copied directly from FormAssembly's support 

public class FormAssemblyFormController2{

    public HTTPResponse res {get; set;}
    public String resBody {get; set;}
    public String endpoint {get; set;}


    public FormAssemblyFormController2() {
        PageReference pageRef = ApexPages.currentPage();

        HttpRequest req = new HttpRequest();
        req.setMethod('GET');

        if(pageRef.getParameters().get('tfa_next') == null){
          //Replace 'FORM_ID' with your form's ID number
//the next three lines are the juicy bit that Patrick Tewson and I figured out.   tfa_2675 is the field on form2 that I want filled in by the lead.        
          endpoint = 'https://www.tfaforms.com/forms/view/4618890/?tfa_2675='+pageRef.getParameters().get('leadid');
        }else{
          endpoint = 'https://app.formassembly.com/rest' + pageRef.getParameters().get('tfa_next')+'&tfa_2675='+pageRef.getParameters().get('leadid');
        }
        req.setEndpoint(endpoint);

        Http http = new Http();
        try {
                //Execute web service call here
                res = http.send(req);
                resBody = res.getBody();
        } catch(System.CalloutException e) {
            //Exception handling goes here....
            System.debug(e);
        }
    }
}