The Chekin invitation URL will be stored in an info item called CHEKIN_URL in the booking.
This can be sent to the guest by auto action and template variable.Check this example for more information [https://wiki.beds24.com/index.php/Auto_Actions#Send_Remotelock.2FLockstate.2C_Nuki_PIN_or_Chekin_Link_2_days_before_Check-in_.28Self_Checkin.29_-_Select_from_the_predefined_template]
The text of the info code CHEKIN_STATUS tells you the status of the booking in Chekin.