@Kinda Loutfi There are two steps mentioned in the document that points to a different document.
The first step in the document is to Add a certificate .pfx file to your API Management instance which points to this document where you first upload the .pfx certificate from the portal or using Create or Update APIM Management Create Certificate REST API. The API document does list the parameter/sample request/response from the respective APIs. If you are looking for authenticate the Azure Management API then you need to refer to Azure REST API reference document which has the details on the setup the first time when you want to call any Azure Management API.
The Gateway Certificate Authority - Create Or Update REST API does cover the URI definition and parameter that is used while making the request. Please refer to the sample request . If you are facing any issue/error in building the request or any error then please let me know.