Give mailbox permissions to another user

Hi all,

After successful installation, I want to send mail from another mailbox (as indicated in step 8). But the link to ‘active users’ (https://admin.microsoft.com/AdminPortal/Home?#/users) does not work, this just shows a white blank screen. Does anyone know the correct steps to perform this correctly?

Thanks in advance!

The page I need help with: [log in to see the link]