Install for a specific, non-admin, user?

In Catalina (maybe earlier) Installer.app has an option Install just for me (or something like that, my translation from a non-English version) but most of the time it is disabled (in fact, I think I never have seen it enabled).

I would like to use that option, for example when installing something downloaded from an unknown website, and install in a non-admin user account (or even in the Guest account = everything is deleted when logging out) as a basic security measure against malware.

Can I somehow force enable this option?

I see no reason for it to be disabled for most userspace software. Every user has their own Library directory and well written applications should anyway look in the current user's Library before /Library or any other shared area on the drive.

The usual reason for using a .pkg is that the app requires support files in specific file locations. An installer package will only allow the option of user-domain installation if the developer has expressly set and catered for that option.

With the latest security features in Catalina (and Mojave to a lesser extent), I would suggest that the difference between root level and user level installation (as far as limits on malware go) is minimal.

However, you could try using Pacifist, the third-party installer app, which allows you to browse the contents of installer packages and install to specific locations. Note that doing this may cause problems with pre- and post-install scripts contained in the .pkg file, which may expect files to be in particular places.