Hi @Martin_Hill Apologies for the delayed response!
I assume you want a snippet like this so that it can be directly triggered into the browser address bar by typing tb<space><snippet-shortcut>:
{formtext: name=message_id; default=<randomidstring@url.com>}https://mail.google.com/mail/u/0/#search/rfc822msgid%3A{=urlencode(message_id)}
Note the key part is the urlencode function which correctly encodes the angle brackets so they're valid in the URL.
You can also make it so that the message ID is pulled directly from an open Gmail page - so the user doesn't need to copy/paste it:
{message_id={site: text; page=https://mail.google.com/\*; selector=.message_id}}https://mail.google.com/mail/u/0/#search/rfc822msgid%3A{=urlencode(message_id)}