How to have optional fields in a dropdown

Question

Can someone tell me how I can have an optional value in a dropdown menu with mandatory values. E.g: There are main values that should be available in a certain dropdown, and there is one optional value that should go to the same dropdown cuz it falls under the same category. the users should be able to use that optional value together with the mandatory values in the field so its a bit challenging.

There is a dropdown named Order Instructions (A,B and C) which itself is optional and below that is another Order instruction (D) which has been provided as a checkbox since it can be enabled together with any order instruction in the drop-down. But I am to add this to the dropdown because it doesn't make any sense to have an order instruction outside of other order instructions. However, I still have to think of a way to have it with the current behavior.

Answer 1

As I understood the optional option makes only sense if a mandatory option is chosen. Something like an additional order instruction.

If so, the combination of the mandatory and the optional instructions in one dropdown will not work.

If my assumption is correct, what I would do is to show the dropdown, let the user choose one of the mandatory options and only then show the additional option (in form of a checkbox probably below the dropdown).

If I would hide and show the optional option or disable and enable it would depend on the content.

Answer 2

Operating on the assumption that mandatory values means "user must select exactly one option" and optional value means "user can select none, one or more options":

This is the domain of radio buttons. Instead of having a dropdown with 3 options and a checkbox, you have this instead:

Order instructions

🔘Option A
🔘Option B
🔘Option C

☑️Option D

If you have too many options for radio buttons to be viable, you can also use a dropdown with the checkbox right underneath it. As long as it's clear that these two go together, it's fine.