How to connect two peer by the third tailscale node

Tailscale version
Your operating system & version

I have three nodes, a, b, and c, the a node has a service to serve but is behind a Symmetrical NAT, and fortunately, b is behind full cone NAT so b is possible to directly connect a and does not need a relay node.

Now the c is not full cone NAT, it’s port-restricted NAT, and could not establish a p2p connection with a. It would go to the relay node and have a heavy delay.

How could I set the c should via b to connect a, so c would have a lower delay?