How to add field nationality from Custom Entity Country that have a column name nationality from a contact form ?

I have a trouble to setup a field on a custom contact form.

I want to have a nationality field in my custom contact form  that can display a dropdownlist (or lookup value) of all nationalities from Entity country on the field nationality.

at this time I can only create a lookup field in custom contact form, where I can only see the list of country name.

if someone can help me,

thanks,