It seems I missed some documentation from MSFT regarding header. Is also seems I was not setting up properly postman while sending certificates.

According to documentation the header where I can see the certificate is 'X-Forwarded-Client-Cert'. I'm also adding here another link regarding the details of this header for future reference.

I do have now an issue with the certificate chain which seems to be different vs the one in the file, but this is outside of the original question.