Hello all together!
Actually the translation from cURL to PROC HTTP in the part "Working with authentication" is not working in my example.

By putting all information together in the in= - statement like
in="grant_type=password&username=userid&password=mypassword&clientid=user&clientsecret=passwort"
instead of
in="grant_type=password&username=userid&password=mypassword"
webusername="clientid"
webpassword="clientsecret"

it worked.
It is confusing to use -u "app:mysecret" in the cURL command and
webusername="clientid"
webpassword="clientsecret"
in the PROC HTTP statement for the authentification anyway or did I missunderstand anything there?