Issues to add activity parties to custom entities

I have a bunch of custom activities that have activity parties. I'm trying to add the activity parties in a Insert step, but scribe says that the "ActivityTypeCode" field is invalid. Activity Type Code = the object type code of my custom entity.

If I check the "ActivityTypeCode" field, it only shows me the oob values, not the custom ones.

If I do not map activitytypecode, Scribe says Its a required field.

Is there something I should be doing differently?