Validations on PO action button Validate

Summary:

Hello,

Client needs to restrict POs to be created with suppliers having a specific supplier type only. If supplier type validation fails, user wants to see a popup error/warning message in either of the two ways.

1. While validating the PO from Actions -> Validate, popup message should appear and PO validation check will fail.

OR

2. Popup message should appear after clicking the Submit button on PO screen, but before the approval workflow initiates. In case of validation failure, PO should not be submitted for approval.

Please suggest if it is possible to add custom validation rules for Validate action. Alternatively, can we trigger any custom validation logic at Submit button click on the Create PO screen?