I had the same problem with Origin! I don’t know if the issue was with the google capcha system or how the app works. I had the same problem when login from outside the app, in a normal web, so I suspect it’s the first option.
I had the same doubt as you. After searching on google, the answer is ‘put whatever you want, Valve don’t check the validity unless you do a commercial project that spam them’.
I thought I had put it more clearly, sorry.
edit: it’s strange it doesn’t have linux version, given that is open source, but it’s done in .net/wpf, I guess that’s why. That said still should be possible to be multiplatform in the future, the code is in github.