OIC: Establish connection to GMAIL using REST Connector

I am trying to establish a connection to the GMAIL REST APi using the OIC REST Adapter, however I am having issues setting up the REST Connector.
First I tried setting up the connection using "OAuth Authorization Code" as the Secruity Policy which initially worked fine. But then I realized that I need to redo the "provide Consent" Action every hour (basically always once the access token expires)

After some investigation and testing, I found out that the GMAIL REST APi does not provide a refresh Token out of the Box. It is required to send additional Parameters in the Authorization Request, access_type and prompt (before the Access Token request). Based on the OIC Doucmentation (https://docs.oracle.com/en/cloud/paas/application-integration/rest-adapter/oauth-protection.html#GUID-1F374ED3-297C-42C4-9507-79971A32FAA2) this should be possible using the "OAuth Custom Three Legged Flow" Secruity Policy. However this is were I got stuck. I followed the Documentation as per my understanding and setup the connector but when I try to perfome the "provide consent" action i get the following Error Message from the Consent