API Endpoint on LTM

Hi

I'm trying to come up with a solution to meet the following flow/requirement:

1. LTM presents a VIP that is an API endpoint for a web server
2. Client requests authentication using json body, e.g. https://1.2.3.4:443/api/auth  -H 'Content-Type = application/json'  -d '{ "username": "testuser", "password": "testpwd", "clientContext": 1 }'
3. APM does LDAP authorization on behalf of the web server using the username and password in the body
4. Once authorized, F5 provides token/cookie to client for further API calls.

I have tried various combinations of solutions but cannot find a way to do this simply. Looked at API endpoint protection, APM policies, iRules, can't seem to get this simple use case working...

Thanks

Mick