Thanks Sajid,
The first link does what I want to do - but whith a different starting position.
I don't get the username with as a parameter, but as NameID (or subject, different field with the same content) of the SAML Authnrequest.
However, the idea of setting the username field readonly, is good! :)
So, follow-up question: Any irule-Magicians around who can help me to extract the nameID parameter?
(What I'd need is something like ACCESS::saml authn [value] but with the f5 as IdP, not as SP... ;)
Cheers,
HP.
P.S. For my own reference:
https://support.f5.com/csp/article/K07535385