You should see a new plus icon at the end of the table:
Then a new modal will pop up like this👇
after you enter the name of the field, and choose the type of it, it will allow you to add your custom dropdown options
You can add as many options as you want.
After you hit create, the field will automatically show up in the table as a new column
and when you go create a new source code, the field will show up in here
