Help create a behavior to make custom field B required only if a specific value is selected in anoth

[Abe_Roba]

I want to create a behavior upon issue creation, where I want to make custom field Y required only if a specific value in custom field X is selected.  Let us say custom field X is Defect Type and custom field Y is Defect Subtype.  I want to make the Defect Subtype field required only if DB Script value is selected in the Defect Type field.

How should I write the script?