mandatory option selected

[marcoas]

(Translated by Google)

I would need to be able to configure that it is mandatory to always leave an option selected.
While I want to be able to write to the INPUT (as if it were a standard DATALIST ), I would also need to force the user to ALWAYS choose an option (as if it were a standard SELECT )
In other words, I cannot allow the user to leave something written in the INPUT that DOES NOT EXIST in an option.

How can I do that?

Thank you