Fetch custom field guide instructions not matching

I’m trying to follow this guide:

However, step #2 is not possible. In my “Fetch From” field, I am unable to enter “supplier.gstin” like in the guide instructions and only get to choose from the drop down list and “supplier.gstin” is not available. Should the guide instructions be updated?

I found a working video guide from Fetch From Doc_type - #4 by NCP
I wished the written guide was updated to the correct instructions.