The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.
https://links.advice.hmrc.gov.uk/l/eyJhbGciOiJIUzI1NiJ9.eyJidWxsZXRpbl9saW5rX2lkIjoxMDEsInVyaSI6ImJwMjpjbGljayIsImJ1bGxldGluX2lkIjoiMjAyMDA0MjMuMjA1MzE4MjEiLCJ1cmwiOiJodHRwczovL2F0dGVuZGVlLmdvdG93ZWJpbmFyLmNvbS9ydC84NzI4MTgzODQyOTAxNzYwMT9zb3VyY2U9QXByLUhNUkMtRENTLUhUSlJTLUFnZW50cy0yIn0.3fKKEJ3PgqFMeY0RVdyIyqcjTMeuSGi3doNym3HSyYs/br/77747084137-l