Hi Pedro,
Good day to you,
We have validated the reported query. The Cmd key is not considered as an combination key, to be captured in the browser using the JavaScript. Check the below StackOverflow link for reference.
Instead you can apply, Underline for the text using the Rich Text Editor executeCommand for the corresponding keycodes when the Cmd button is clicked. Check the below sample for reference.
Please get back to us, if you face any difficulties,
Regards,
Indrajith