I know how to put my .pkg installer into a .dmg file,
but when downloaded is just opens the dmg into the finder.
How do you make something popup automatically that alerts the user to start the .pkg file?
That’s the first preference I turn off in Safari 
I wouldn’t like that at all… So I hope (and guess) that’s not possible.
What you see quite often is a custom Background Image in the opened Volume, which contains a quick explanation, e.g.: “Doubleclick the Package to Install XYZ”.
It is not possible AFAIK. Besides, it would be quite dangerous, as some evil souls would use it to launch malware.
Note that if your DMG or the Pkg are not signed, the finder will very much frown under High Sierra.
You can sign your Pkg under App Wrapper, as well as the DMG.
This is what I am trying to do.
Some popup a screen that says double click below to install the package and it’s a big ICON.
For my dmg it shows as a volume in finder and if you click it, it just shows the .pkg file.
Most of the programs I install have a big popup showing the icon and and arrow to drag it to Applications.
I guess we are talking about different things because most program I download have some sort of pop up telling you to either drag to Applications or double click to install the package.
I think what you are looking for is https://www.araelium.com/dmgcanvas
That will allow you to style the background of your DMG window.
I do not know araelium, but if you only need to set a background image in a dmg file, you can do that int- the Finder.
Simple, easy and free of charge.
