Smartsheet URL and Redirect URL for connection with Celonis

Hello together, i am trying to connect Smartsheet with Celonis. I already have a Client ID and Client Secret but everytime I try to connect both, Smartsheets shows an error saying Redirect URI ist missing or wrong. Does anyone know what to fill out in the Developer Settings of Smartsheets in the field APP URL & APP redirect URL ? Thanks in advance

I have filled in an APP URL (something like https.// and an APP redirect URL ( and every other mixture of URLs but nothing seems to work out. I have also reviewed various Smartsheet Docs but found no answer until now.