FileMaker new Node-Red user

Hello all,

Apparently, I just can't grasp how to simply make a web hook, and whatever output that delivers, how to put that in a field in FileMaker. Ideally performing some FileMaker script after data is retrieved.

So I'd say I go from a HTTP IN node (Method POST) directly in a FileMaker Create Node. But if I learned correctly, I should first form the data (with the Function node) so that the FileMaker Create node can receive the data correctly.

Content of Function:

var fmpayload = {
    "MollieCallback": msg.payload
} = fmpayload

return msg;

What I try here, is to put all received data from msg.payload, to the "MollieCallback" field, on the specified layout what I entered as a string in the properties field of the FileMaker Create node.
I get an error in the FileMaker Create Node that the Field is missing, although I though I specified it in the code?
I'm not sure if (and why) I should use the JSON option at the Data field of the FileMaker Create node, with '' entered, or use the 'msg.' option with '' (see screenshots, a picture says more than this whole text)

Hope you can shoot me in the right direction. Thanks!