With custom map i mean a historical map overlay,  i thought i could put this in a resource map.

No idea if this works, playmaker used a GUI texture and i had to import the map coordinates + map size. It had some limitations so i bought this asset to have more controll.

«Custom map», you mean «Provider - Custom»?
In this case, specify the link to the map, which you want to use, and URL, which you use.
Usually there are two possible problems:
1. You have specified incorrect URL.
2. Incorrect response from the server. Expected PNG / JPG, 256x256.

It looks like this is not working. the map turns grey and i can't get it working. I'm now wondering if this is possible or not.

The idea is using a historic map, i have all needed coordinates, but no idea how this works.