BTW why do you want to give access to admin@system-domain, this account should be used only for SSO configuration purpose? If you want to try then
login to the vCenter server(vsphere client) using any of the admin account.
Select vCenter
go to permission
Right click add permission
Click add
Select "System Domain " in the domain
Select "admin" from the users and group and click add and do OK
Provide appropriate permission in the right panel
Note: This is not recommended from VMware. Better use a domin account for loggin in. This just for your information purpose
