X++ disable fields base on another field with lookup option

Here is the challenge I'm facing:

Main Form X has a field(string type) called Y & Y filed has a lookup with data entered(CAN 2, CAN4, CAN6) created separate table & form for this Y lookup/drop-down)

1. When user clicks on the drop-down lookup menu on Field Y and choses value 4 or 6, 3 other fields, lets just say Field K, Field L, Field M must be disabled. How do I write X++ code for this and what type of override method? Do I override it on Form method or Data source's individual  field method?

Form>method>override>Active method? or Form>DataSource>Fields>Field Y>override modified method?

Thanks!