All you have to do is, once the extension is open and locate the shortener you want to use, display the options of the shortener with the secondary mouse button and choose "Shorten with aliases".
Not all shorteners offer this possibility, but all those who offer it have this option in UMS.
A new panel will appear in which you can enter the desired alias. If that alias is not available, the action to be taken depends on the shortener.
Some shorteners send the short link with a new random alias, while others throw an error. In the latter case it will be necessary to reuse the option and choose another different alias.
Note: Pending approval of the latest version, this option may not yet be available in all versions of the extension.
