Thank you for your answer! I just figured it out.
I copied the request body out of the documentation and didn’t recognize the missing quotation marks. I tried it by adding the key and token to the body just like in the custom fields value set, update and remove request.
It worked!
So my Request looks like:
URL: https://api.trello.com/1/customFields
Body:
{
“idModel”: “”,
“modelType”: “board”,
“name”: “priority”,
“pos”: “bottom”,
“type”: “text”,
“display_cardFront”: true,
“key”: “”,
“token”: “”
}