Similarly to XDS, the MHD profile is using the same registry/repository principles. The difference is that it is much easier to implement as no complex SOAP syntax is required.
In order to query the registry, MHD is using some of FHIR _search principles (for more information see https://www.hl7.org/fhir/search.html).
The query consist of a http POST to https://api.qa.brusafe.be/mhd/DocumentReference/_search
In the body, you will need to set an x-www-url-encoded parameter which can be construct as such :

The value you set there should always have the form :
1.3.6.1.4.1.21297.100.1.1|<Patient_National_Registry_Number>

In the headers, you will need to specify the content-type and the JWT authorization token previously retrieved with the prefix 'IHE-JWT' as such :

Another type of authorization could be used, the SAML. If you encode the SAML in B64, you can use it in the header similarly to the JWT but you'll use the prefix 'IHE-SAML' as such :

The response should be a FHIR Bundle of search results, each entry correspond to a document. It is important to note that all documents have an id in the DocumentReference resource. This id will be useful later upon retrieving.