Hello and welcome to our forums,
Unfortunately our "Default Program" feature can be used only for application which are delivered and installed by the current setup project (files from the package).
To set as default program an application which is already installed on target machine you can try to wrote your own custom action.
https://www.advancedinstaller.com/user- ... tions.html
https://www.advancedinstaller.com/user- ... tions.html
If there is anything else we can help you with just let us know.
All the best,
Daniel