Where can I find the "Key" name for Recipe Builder to update project properties in GCX?

I am working on a Recipe to update a custom field value with the value from an integrated system. The recipe step to update project properties is asking for 2 fields - a Key and a Value. I am assuming the value in the field from the integrated system but the key is not working when I use the Custom Field Name. I was able to find the custom field id in the connectors section of recipe builder but the “key” is not part of the data that comes back when I test the code to “List custom fields”. Where do I find the key references for this step?