A developer here kindly pointed out that Xenforo 2 now has custom thread fields that can do this. So I am going to try that out first. Thank you to everyone who took the time to look at this.
Is it vital that the same forum (node) allow for posting both normal threads and threads using the template?
If the answer is "No" then based on your screen shot I'd suggest trying to avoid any add-ons (to avoid future upgrade issues) and just use custom thread fields for a designated forum.