Just setting up the SFTP connection by selecting the key and entering the password will fail (bug). Tested with version 3.2.2. Instead some addtional steps are required:

  • set up ssh with keys as usual How to set up ssh key-based authentication
  • then create .ssh/config on the mac (local client) and fill in:
    Host *
     AddKeysToAgent yes
     UseKeychain yes
    
  • ssh-add -K /path/to/private_key to add the password of the private key to the keychain
  • then set up connection in Forklift

How to store the ssh password in the keychain

terminal - macOS Sierra doesn’t seem to remember SSH keys between reboots - Ask Different