Should I just run it and it will install itself in a toolbox (this is how things are installed with flatpaks, right?) or will I have to create a toolbox manually first? or will it simply not work with silverblue?
Applications doesn’t exist only in Flatpak or RPM format (and so on), but also in an executable format bundling all required libraries and so on (I’m not an expert, someone can correct me: statically built, the executable doesn’t rely on the version of libraries available on system).
In your case, just run tresorit_installer.run and it will install the application in /home/user/.local/share/tresorit directory by default. No need to use a toolbx container.