How can I install a non-notarized application that is not in the App Store and not from a verified developer?

Is there a possibility to install software that was not "notarized" for macOS Catalina? I have an application that worked on macOS Mojave but cannot be installed anymore on macOS Catalina. I think the developers must "notarize" this software. Checking with spctl -a -vv my.app confirmed that it is **rejected**. 1. What can an end user do about it to install it anyway until this process is finished?