How to modify/replace standard Error Message for source agreement field on PO

Summary:

Need to modify/replace the standard Error Message "You must enter a value" for source agreement field on PO

Content (required):

Need to modify/replace the standard Error Message "A value is required You must enter a value" for source agreement field on PO. I see there is "Manage Messages" feature but I do not see any seeded error message for source agreement field in that. Need help how to create a new custom message or any documentation where I can refer to would be great.

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