There are two different APIs for WidgetMakr, the Payment API and Conversion API.
Conversion API: this API pulls data that has been processed through WidgetMakr widgets. Directions for accessing the documentation on this API can be found here.
Payment API: this API allows you to create custom forms as an alternative to using a widget. In order to use this API, it must be enabled on the individual widget from your WidgetMakr account. You must also contact the WidgetMakr Help Desk to enable the tokenization API for your domain. Otherwise, this API will not work.
Directions for accessing the documentation for this API can be found here.