M2M access token https request certificate

I am currently working on a M2M project.
My client embedded application is requesting an access token through the https://…us.auth0.com//oauth/token endpoint with a proper payload.
Since I am in an embedded minimal world, I need to supply a certificate as without one, no poper validation is done. Everything works fine if I don’t use a certificate but when I try to use the one supplied in the Machine to Machine application, it fails.
Where can I get a proper certificate for this endpoint?