How to make a particular field mandatory on OAF Page

When new employees are created from our custom 'HR Manager' responsibility for Sweden, the local unit field is supposed to be populated (See screenshot below). Since this field isn't listed as mandatory, we're interfacing blanks, but the field is mandatory in Oracle Forms. Without this field populated, we cannot navigate to salary or other areas of the assignment screen. How can we make this field mandatory and also throw a warning if they try to advance without populating this field? This field translates to soft_coding_keyflex_id on the assignments table.

Any help is much appreciated!