Include form data in email subject

This simply must be mentioned somewhere, but I just cannot find it in the documentation or after 30 minutes of searching the forum and issue tracker.

I want to have my form send a custom subject line that includes a field value in it so that subject lines don't get too repetative and confusing. For example: "Inquiry from %field[full_name]" would insert the value of the "Full Name" field, producing "Inquiry from John Doe".

I thought %post might be the right token to use, but I wasn't able to make that work either. Any suggestions on how to achieve a dynamic subject line?

Thanks