OAuth2 redirect URI

Hi, I am trying to establish an OAuth2 connection via keycloak sso. I can reach the realm's site, but then I get either "Wrong Redirct URI" or " too many redirects".
What I have done so far:

auth_config.php
$xerte_toolkits_site->altauthentication = "Oauth2";

library/Xerte/Authentication/Oauth2.php (just removed the .dist)

library/Xerte/Authentication/Oauth2_config.php
$oauth2config = array(
'CLIENT_ID' => 'myclientid',
'CLIENT_SECRET' => 'mysecret',
'REDIRECT_URI' => '??????????????????',
'AUTHORIZATION_ENDPOINT' => 'https---sso.mydomain.de/realms/myrealm/protocol/openid-connect/auth',
'TOKEN_ENDPOINT' => 'https---sso.mydomain.de/realms/myrealm/protocol/openid-connect/token',
// 'ACCESS_TOKENTYPE' => OAuth2\Client::ACCESS_TOKEN_BEARER,
);

I would appreciate any hints what else needs to be done and what link must be given for redirect.
PS I also use caddy as Proxy with TLS
PPS: read --- as / .