First, make sure you understand that you give-up any security, when you try to bypass SSH host key check. You effectively lose a protection against man-in-the-middle attacks.
Anyway, you can use use SessionOptions.GiveUpSecurityAndAcceptAnySshHostKey
.
It's NOT recommended though. You should always set SessionOptions.SshHostKeyFingerprint
.
Learn how to determine SSH host key fingerprint.
See also Suppress the use of host key in SFTP or SCP using WinSCP.