Thanks for the question! Can you provide the full request that you are sending (including endpoint)?

As I read the documentation,

This usage is generally intended to be part of an email workflow and does not require authentication. The changePasswordId used on this API request will have been previously generated by the Start Forgot Password API or by using the Forgot Password workflow on the FusionAuth login page.

You will want to make sure that you are including a changePasswordId in the request. Additionally, the link that you posted is part of a multistep process. Can you confirm how you are attempting to architect your request? Is this being used as part of a Forgot Password workflow or Start Forgot Password API?