how to autofill my contact form

You would need to use a choice method or javascript and have all of the zip codes and cities. I believe there are services that can do this for you.

Agree with Josh you would need to do this with Javascript. The services are typically used for populating multiple fields such as company name, number of employees, city, etc. so that your lead only needs to provide basic information like email address. 

This is a javascript solution for sure!

I know i can do this with the help of jquery and javascript,but is there any marketo way of doing it from admin?..