Hi @TimoToothousand,
Could you please share a screenshot of your flow's configuration?
Do you want to fill a dynamic content within Parent List Id field of "Create a card" action to create a new card in Trello?
Further, could you please show a bit more about your SharePoint list?
I agree with your thought almost, the Parent List Id is encrypted. Actually, the "Create a card" action of Trello connector find the specific Board and Parent List via a series of string which likes a GUID string, such as, the '59ed4de0effc7115f87234d7' represents the Board Id and the '59ed4dfbd6d206f46ca2c712' represents the Parent List Id, the "Create a card" action UI just translate the string (e.g. '59ed4dfbd6d206f46ca2c712') into a friendly name.
If you want to fill a dynamic content within Parent List Id field of "Create a card" action to create a new card within Trello, please make sure that the Dynamic content filled within Parent List Id field store the encrypted string (e.g. '59ed4dfbd6d206f46ca2c712') . I have made a test on my side and please take a try with following workaround:
The flow works successfully as below:
Best regards,
Kris