Hey @matt p11​! While there isn't an "official" hotkey, our Support team use a pretty nifty workaround for this.

Step 1: Create a Saved Reply that only contains the first_name variable.

Step 2: Name the Saved Reply something you'll remember - our team use "fn".

Step 3: In your next Inbox reply, use the shortcut #(name of your new Saved Reply).

Step 4: There you go! The customer's first name is added.

I've recorded this GIF to show you how Steps 3 and 4 look. Let me know if this helps!

Love that solution, thanks Eric!