Run Signal and press Ctrl-Shift-I to open Developer Tools (it's an Electron app). Switch to Console tab and enter the following command:
Use your actual phone number instead of +123456789.
If you prefer voice mail verification, use this command instead:
Things got considerably worse in the past years. The process of setting up a standalone desktop version of #Signal got more complicated and is broken even in the development version where it is enabled. There is a pull request to fix this which itself became broken.
A Mastodon instance for info/cyber security-minded people.