B4J Question B4jPackager for MAC - how to include Developer ID Certificate

I can make a DMG package of my B4J project on my MAC OK.
ie. using MyB4Jpackager (wrapper for javapackager tool)

I can click on the dmg file and install it ok.
It even runs OK if it all done on the same mac.

But, when it is downloaded from my web site, it won't run due to an unknown developer.
I need to include a Developer ID certificate.
I am using JDK8 (can't commercially use JDK11 - I believe!).

I think I can use my Apple developer account to create a Developer ID certificate.

Can anyone advise how to include this certificate in to the dmg file?