Can a Trigger Action Fill a Text Field with Part of a Message?

I'm using a non-Zendesk email form for customers to submit tickets. The form ensures details about the relevant product are included in the body of the email that creates a ticket. Some of those details need to get into custom tickets fields, which my agents are currently copying and pasting manually. Is there any way I can use a trigger to fill these? I know I can set a trigger action to fill my custom text field with preset content, but I'd rather fill it with whatever is in that specific predictable spot within the the ticket comments, like a placeholder.