Can you please provide an example of ReceiveAuthnRequestByHTTPPost call?

The IdentityProvider.ReceiveAuthnRequestByHTTPPost is part of the SAML low-level API.
We recommend the use of the high-level API wherever possible as it’s simpler to use.

out authnRequestXml, out relayState);

The Request is the HttpRequest object in the page’s context. The authnRequestXml output parameter receives the SAML authn request XML and the relayState output parameter receives the optional relay state.
You’ll find an example of the API call in the SAML2IdentityProvider project under the Examples\SSO\LowLevelAPI\SP-Initiated folder.
The SAML/SSOService.aspx page demonstrates receiving an authn request using various SAML bindings including HTTP-Post.