Creating Leads in Marketo using REST API

I am trying to create leads in Marketo using their /rest/v1/leads.json endpoint. The process is working, but will not save 2 custom values for the lead - but the Name, email, phone etc are all getting saved for the new lead. The form is not a Marketo generated form, and I ran across this...

"The Munchkin admin page also allows you to enable the Munchkin API. With the Munchkin API, you can integrate third party software with Marketo and your web site; do custom tracking of events such as multimedia playback; and use your own, non-Marketo-generated forms to cookie users and/or create new leads in the Marketo database."

If I am using a non-Marketo-generated form - do I have to use the Munchkin API? I also noticed that there wasn't much in the 'field mapping' area of the admin for these custom fields - but I'm not sure if that matters when creating the lead or not thru the API.