I believe 500 error code is sent by the server you are making that http call to not from Integration Server. That means the API is expecting that JSON object in the URL. But I doubt anyone would expect credentials in the URL. Since you already mapped your Authorization, that part should be working. Try removing the extra information from the json string you are creating, you can just drop them before creating the string. If it generates an authorization error then check your auth parameters.
By the way I still don’t see anything related to Authorization in the url. You have added these to the JSON object yourself.
“ssaCompositeRequest”: {
“ssaCompositeIndividualRequestArray”:
You are sending this json object in the URL. These 2 fields don’t look like they are related to authorization. Your first entry was about removing requestlink?string={}Authorization={}. So far I am a little bit confused. This is the expected behavior. 500 response code is generated by the end point you are calling. Everything feels like out of context so far.