How can i make required custom field only for few projects.

Hello,
I have 1 custom field and 5 projects. The field name is: "xy", the projects name: "x1", "x2", "x3", "x4", "x5"

I want that the "xy" custom field will be required only for "x1" projects, in the "x2", "x3", "x4", "x5" project the field need to visible, but not required.