Why is it that I can select the Add Custom option on a checkbox but not a select field? Seems to be similar logic and a valuable option.
Radio buttons also have this. They are very similar. More than likely because the author has just not gotten a chance to add it to the select field. You can try contacting him and making the suggestion. I’m sure it’s on his to-do list. https://www.advancedcustomfields.com/contact/