Rust and C are pretty much the same for the No framework example. You simply need to open a socket with the IPPROTO_MPTCP protocol. For that you can use the Socket2 library see here.
If you plan to use a more complex library (HTTP library, Web framework, ...), it may not be as straigthforward as just using Socket2. Let's take Hyper for example, they support differents IO type, allowing you to create a Socket2's socket and pass it to Server::from_tcp.