Sign in to Sendana the way you usually do, tap your profile icon in the top right, then open "Security Settings" and choose "Add a passkey". Your phone will ask you to confirm with Face ID, Touch ID or your passcode, and that's it. Next time you sign in, tap "Sign in with passkey" and you're straight in.
One thing to know: the "Sign in with passkey" button only works once you've set a passkey up. If you tap it before you've actually set up your passkey(s), it will not work.