So this may or may not help.
I recently found out that the InfusionSoft API calls for updating Custom Fields based on the ID of the custom field.
You can GET the Custom Fields using the InfusionSoft API / docs found here:
https://developer.infusionsoft.com/docs/rest/#!/Contact/listCustomFieldsUsingGET
Basically you will be able to authenticate using your InfusionSoft Dev-portal account, along with a valid InfusionSoft account / credentials.
Create one if you don't have one yet: https://developer.infusionsoft.com/
Once you have your details you an log in at the above API page and then issue sample API requests directly from the documentation page (to see how IF would respond to the query using the InfusionSoft account you authenticated with).
When it DOES respond, it will have a list of Custom Fields along with the ID number which you can then turn around and use on the POST / Create API endpoints.
Not sure if that will help since this question seems to be more about a specific library, but I figured I would post it for anyone who is looking to use Custom Fields along with the InfusionSoft REST API.
Oh!
One more note.
POST Requests
When you do a POST to add or update data with a current client, you will need to submit an array of custom Fields, each of which will have ONLY two properties:
{
"id": {whatever ID number for the the custom field you want},
content: {Whatever value you want to assign to that custom field}
}
Every item in the array needs to have BOTH of those properties and DOES NOT refer to the Custom Field name at any point in the process. It's all by ID number and the value is ALWAYS set by the content property.