Authentication failure when setting up remote python SSH interpreter from MacOS

I'm using macos and PyCharm 2018.2.5 professional.

By macos terminal, if I connect with the command

    ssh -i p_k.ppk user@www.xxx.yyy.zzz -p 22

everything goes fine (after typed a passphrase) and I'm able to access a remote AWS machine.

If in PyCharm I fill the fields of Project Interpreter -> SSH Interpreter -> New server configuration -> Key pair (OpenSSH or Putty) I instead get the error

    Connection to user@www.xxx.yyy.zzz:22 failed:

    Authentication failure

Does anybody know the solution? Thanks!