I believe I an following the instructions correctly.
POST https://sandbox.momodeveloper.mtn.com/v1_0/apiuser HTTP/1.1X-Reference-Id: f9b22c29-eaeb-4423-9a29-960ece987648Content-Type: application/jsonCache-Control: no-cacheOcp-Apim-Subscription-Key: ••••••••••••••••••••••••••••••••{ providerCallbackHost: "string"}The X-Reference-Id is a UUID from online generator.Ocp-Apim-Subscription-Key is from mu Collection product.The result:HTTP/1.1 200 OKcache-control: no-cachecontent-length: 247content-type: text/html; charset=utf-8pragma: no-cachevary: Originx-content-type-options: nosniffx-frame-options: SAMEORIGINx-xss-protection: 1; mode=block<html><head><title>Request Rejected</title></head><body>The requested URL was rejected. Please consult with your administrator.<br><br>Your support ID is: 11597720049319855937<br><br><a href='javascript:history.back();'>[Go Back]</a></body></html>