I've got a contact form with all the usual suspects: name, email, enquiry.

When I send as an non-logged-in user, the 'from' address that appears in my email is the site AdminEmail I defined in my _config file...

But how/where do I write the code that will take the submitted email address, and use that as the from address?

I'm just using the standard ContactForm page type - do I need to extend that with my own pagetype and add some code in there, perhaps?

I wanted to do the same thing. You can't do it with the userdefined form, you must write your own code.

I've attached how my file turned out. Be sure to make your email template in mysite/templates/email named ContactPage_Email or whatever you want to change it to.

The code isn't commented, so if you don't understand some things, let me know.