You need to create two sets of drop down menus in Studio.

1.) Create your parent dropdown (Country), name it as required, add your values and save this.
2.) Create your child dropdown (City) and do the same for this.
3.) Navigate to “Leads” and add a field with data type “Dropdown”.
4.) Name the dropdown and select the PARENT dropdown list.
5.) Save this.
6.) Add another field to your Leads module, but this time the data type needs to be “Dynamic Dropdown”.
7.) Name this as required and then select your CHILD dropdown in the “Drop Down List”.
8.) Select the parent dropdown in “Parent Dropdown”.
9.) Save this.
10.) Add both of these fields to your list/detail/edit view layouts and any other layouts you require this to be viewed in your Leads module.

https://suitecrm.com/forum/developer-help/260-dynamic-dropdown-inside-studio