How-To? Required Custom Field

Using RT 3.6.4, I am trying to make a custom field to be required at
Ticket creation time (through the SelfService interface).

This field has the ‘Select One Value’ type.

I set the Validation field to: ‘(?#Mandatory).’ but this did not work.
I could still create a ticket. Reading other threads I tryied:
’(?#Mandatory)(^[A-Z]+$)’. This did not work either (Yes the choices
are starting with an upper case letter).

Could someone confirm that Mandatory means Required?

Is there a way to forbid creation of a ticket unless the required
fields are filled?

Thanks,
Thierry Thelliez