Reverse SSH tunnel to allow others to use my machine as a jumping station

I'm running ubuntu 18.04.

I have a server that I SSH to, but I first need to turn on a VPN for that to work.

I want to enable other users to SSH to a port on my machine, in order for them to reach that VPN. So they ssh to port 12345(example) on my machine, and they are then on the remote machine.

So, a jumping point, I think.

So far, all attempts at using the -R option have failed.

How would I do this? What am I missing?

Consider the following:

• My machine (A)
• The server behind the VPN (B)
• The machine of the person trying to use mine as a jumping point (C).

Both of the following commands have failed:

• On A

  ssh -A -R 19999:localhost:22 B
  

• On B

  ssh -R 19999:localhost:22
  

Edit: The VPN is a standard L2TP one that Windows allows you to set up.