Yes or No in a base form field

How do I limit a response to either yes or no in base form field? I’ve tryed everything I know, including Boolean yes/no, list drop down and combo box.

Ideally, the user arrives at the field which is already pre-filled with “No”. They can then type “Y” which will fill in “Yes”.
If this can’t be done, ok and I’ll use anything that does.
Thank you!

Ps, This is my second attempt at getting help.

Duplicate of Yes or no in base form field