I am half writing this for myself for the future in case I forget the exact syntax. If you want to update a custom field for a contact inside Keap but don’t want to create your own API, there is an easy way to do it. For this scenario below I am using make.com which is a zapier alternative.
In the URL you are going to want /v1/contacts/contact ID. The contact you can dynamically map in make.com. For this example, I am testing on my own contact record, so I have hard coded the KEAP contact ID.
Make sure the method = patch as we are updating a contact record and not creating from new.
I am updating custom field ID 784956. I am inputting the number 7 into this field as it is a numeric field only,
That is the syntax needed to update a custom field via REST API in Keap. Works for max, max classic and pro.