Giving client access to their account if I have set it up rather than them.

Hello - I have a client who I set up with an account and then booked the for an appointment over the phone. Now, this client would like access to their account so that they can book directly and it is confusing.  I know in the past there was a way to for the client to do this by clicking on the Change/Reschedule button in the confirmation email but I either forgot how to do this or the process has changed because when she chooses Change/Reschedule she is sent to the booking page and I cannot for the life of me remember a simple way to get or give her access.