The POST URL for a form is https://<instance>.marketo.com/index.php/leadCapture/save2 where <instance> is, for example, app-ny99.
They also need to include the form's numeric `formid` and your personal `munchkinId` to pair their data with the underlying form.
I hope they know what they're doing as they should've known where to find this if they're used to working with Marketo as a destination for non-Marketo-created forms.