Click Your Account and Manage Servers or click this link: https://ems.element.io/user/hosting
Click Server Admin
Select your host
Click Add user Enter the username and click Add
Select Make new user server admin if you want this user to be able to use the Synapse Admin API to perform administrative tasks on your server. Be careful with this as this option has a lot of power on your server.
Make new user server admin
The username and password for the new user is displayed. You will only be able to see this information once so send the password to the user in a secure manner and then delete it