The Error Code 400 corresponds to a "Bad Request" from the server side. The error arises as there is no Endpoint from MathWorks to authenticate you using a Post Request on the URL you have mentioned.
I understand that you are trying to authenticate by creating a Local IdP, for which you will need to create an YAML file which looks similar to the following:
Kindly refer to the following documentation to learn more about configuring server authentication for "MATLAB Online Server"
Hope this helps!