First, always start with the documentation:
you're wasting your own time trying to guess parameters that don't exist.
The WebDirect URL allows you to specify:
-Return URL "homeurl" (where user goes when exiting webdirect)
This is covered on page 28 of the guide above.
Providing someone with a URL containing a plain text username and password is insecure, that's why it's neither supported, nor recommended.
You can generate a link containing a UUID that corresponds to a record in filemaker, perhaps in a users table. However the account that you use for WebDirect should be an anonymous, locked down login that only allows record access via the UUID you pass as the script parameter when logging in.
Your url would look like:
Your "login" script could just be:
Set Variable [ $$userID ; get(scriptparameter) ]
And then set up your privilege set to grant access only when $$userID = yourtable::youridentifyingUUID
This should at least get you started. You can set the file to log in with guest credentials and add this to the login script:
If [ get(systemplatform) = 4]
Re Login [ webdirectuser / pass ]
That will automatically log you in on WebDirect, or prompt for user/password anywhere else.
You can build on this to make it as complicated as you need.
I apologize for the slow response. Thank you for your guidance. It was the end of a long day and I actually looked at that page and overlooked what you pointed out. I was able to implement what I was trying to do with the client and it worked perfectly. Thanks you for your help.