You first need to plan for authentication. you can use oauth or login into devops and generate a PAT token: https://docs.searchunify.com/Content/Content-Sources/Azure-Generate-Personal-Access-Token.htm
This is how you use in Postman: https://www.sanderh.dev/call-Azure-DevOps-REST-API-Postman/
to upload to a work item you need two operations/apis
First the one on the link you posted:
that will upload the attachment to devops and return a url. with that url you can finally attach to the work item using this second api: https://dev.azure.com/fabrikam/_apis/wit/workitems/2?api-version=5.0
more here: https://learn.microsoft.com/en-us/rest/api/azure/devops/wit/work-items/update?view=azure-devops-server-rest-5.0&tabs=HTTP#add-an-attachment
some more info: https://stackoverflow.com/questions/66101212/ado-upload-an-attachment-to-azure-devops-via-http-rest-request
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