how to dispatch a event and customize the message to show?

I’m using lwc’s lightning-record-edit-form component, as we know there is a <lightning-messages> area to show any error while updating/inserting the record.

However I want to have a custom check the validation of the record and show the lightning-messages when the check is no-good. Is there a way to do so？

for example, if the startDate is later than the endDate, I want to show a error-message in the <lightning-messages> area.

Plz tell me if it’s possible.