If you would need to connect from commnand line to another server using a ssh key pair you would just need to use

ssh -i file.key user@hostname-ip-of-the-server

You might get into an error that prompts you that the file has an invalid format, most of the time when using the key generated from Windows Puttygen. But, you can use open the same private key in Putty, go to menu Conversions and select Export OpenSSH Key (force new file format). Use the new file for the command line above. It should be good to work.

But, if you are in need to connect from Midnight Commander to another server which requires a ssh key, this requires some more steps:

In the file .ssh/config you will need to add this lines, or even create the file if you don't have it already:

Host The.host.to.the.server.or.just.the.IP.Address
_User the-username-here
IdentityFile /path/to/the/file.key

Make sure you start your ssh-agent using the command below. Make sure you use the backquote (`), located under the tilde (~), rather than the single quote (').
```
eval `ssh-agent`
```
Add the key file to the ssh agent using
```
ssh-add /path/to/the/file.key
```
Now, you should be able to use the menu SFTP Link from mc using the below, as you have defined them in the .ssh/config file:
```
the-username-here@The.host.to.the.server.or.just.the.IP.Address
```