Most parishes will have the secretary just type the name of the priest in lieu of an actual signature.
If you know the name of the priest, just ask them to put it on the document.
If you don't know the name of the priest, the original parish should be able to help you figure it out and put the name in. While signatures are preferred it doesn't always work out that way—secretaries do a lot of the heavy lifting when it comes to details.
I would contact the parish where you were baptized and just see what you can find.