This requires the https://github.com/shravan2x/Gameloop.Vdf library that works similar to newtonsoft in that it lets you serialize/deserialize the vdf structure

With this, you need to deserialize the file ,,Steam\userdata\<USERID>\config\localconfig.vdf

Then, in that file go to the UserLocalConfigStore ~ Software ~ Valve ~ Steam ~ apps ~ 413150 group node, and add a new value node called LaunchOptions the value of this node is what steam uses as the launch options.

It should be noted that steam might need to be turned off while the installer edits this file, as steam constantly updates it while its running.

I have no current plans to implement this, but I'd be interested in a pull request if it worked well.