Custome field does not appear as an option for argument in custom function - Issue Tracker Add-On

Custome field does not appear as an option for argument in custom function - Issue Tracker Add-On

Hi there,
 We are trying to create a custom field for integrating our Portal to Gitlab. We created a custom field  for the project ID in Gitlab. However, when editing the custom function, this "custom field" does not appear as an option for assigning it as a "Parameter Value" to a "Parameter Name" in the function's arguments. Interestingly, it appears when editing the Append bug parameters in webhooks.
Is it a bug? Do we need to do something else to have appeared the custom field in the custom function configuration? How can we do that?

Thanks.

Patricio.