Have whomever you wish to add as an alternative host log into their personal Zoom profile page using the webportal: https://seattleu.zoom.us/profile
Next, have them click Show under Sign-In Email.
Have them provide you with the email address that is listed under Sign-In Email.
Enter the email address that was provided to you in the Alternative Host text-box when scheduling or editing a Zoom meeting.
Information from this support article was sourced from https://help.lsit.ucsb.edu/hc/en-us/articles/360042270012-Zoom-Why-Can-t-I-Assign-My-TA-as-an-Alternative-Host-