Submit Lockbox processing

I want to make the GL Date Field Mandatory in the "Submit Lockbox Processing" Window.

Attached are the steps I followed but it is not working. Can Someone help if this can be done and how

Read below in Metalink, not sure if it due to this

Note: Changing a field on a standard form to make it mandatory is considered a customization to the form and thus not supported. These simple steps only serve as an example how the standard form could be customized to make Amt Agreed field mandatory.