For advance search please go to Classiads options > Content field
Every content field have such options to add in the search and advance search like the screenshot you can see below.
but if you also checked the box of advance search then it will go in the advance search and only open when you will click on the advance search button.
Also in the content field you can have a selective categories. If you select a specific category then this field will show in the search only when you will select that category in the search bar.