HelpDesk triggers can detect appearance of some text in the message body using regular expression. Thus, you can check if there is specific text of pattern in the message and then set some value in the ticket field.
Unfortunately, you can’t extract the value from a message with the help of help desk trigger.
I would recommend you to create a Microsoft Flow, that will extract text from the comment message and update ticket accordingly.
All comments are stored in Comments list (/Lists/Comments). You can create a Microsoft Flow, that will be triggered on list item creation in this list. Then check message text, extract value and update related ticket accordingly.
Alternatively, you can request our paid support to implement this feature for you. Just send a request to firstname.lastname@example.org