How do I add a dropdown menu to a form?

I am trying to create a dropdown list that will insert numbers based on numbers already used for that group of numbers. Example: dropdown will include numbers such as (100, 200, 300, etc) and I want to append a group of numbers to them like this (100-0001). The appended numbers need to be sequential starting at 0001 but they need to be sequential according to which prefix is used. Also, I need to insert this dropdown on a form in WordPress.