you need to go to admin panel then click 'listing types'
then then gives you a list of listing types, then click to edit autos,
then once in there there is a section down the page called 'settings'
where it says general category there is a dropdown with the make of a car. This is your master category.

now you need to search for that category in your categories section in admin panel.
when u find that there are 3 options in the list of categories- 'build' 'edit' and 'delete'
click build and select 'build a submit form'
this is where you add your listing field.

Hope that helps
And i am sick of writing the word category LOL