signature in preview

How do I create a signature in Preview? I am using a Mac Mini that does not have camera or trackpad. Apple support says to use the signature button and choose either trackpad, camera or iPad/iPhone. I don't get that option. When I click the signature icon I get this message: A trackpad or iSight camera is required to create a signature. I'm running Catalina 10.15.6 and my iPad Pro is running OS 14.