Set "subject" as a dropdown list on form

If I’m understanding you correctly, create a a “Drop down menu” on your form,

e.g.

[select subject_menu "Subject 1|Value1" "Subject 2|Value2" "Subject 3|Value3"]

Then just place [subject_menu] in the email subject box.

If you’re using Flamingo to store the mail messages, then add

flamingo_subject: "[subject_menu]"

in the “Additional Settings” text box at the bottom of the page.