This returns the path to the native askpass executable which can be used by git-credential or ssh-agent. Most users don't have worry about this.
ssh_askpass()
On Windows and MacOS the package automatically sets the SSH_ASKPASS
and
GIT_ASKPASS
variables on load (if not already set). If these are set
you should be able to run e.g. sys::exec_wait("ssh-add")
and you should
be prompted for a passphrase if your key is protected.