When adding AD as an identity source, you need to enter the Domain Name and provide the credentials of an AD user with domain join privileges to authenticate and add the domain.
In the vSphere Client, you need to select 'Add Identity Source' and choose the appropriate method for AD integration, which is typically 'Active Directory (Integrated Windows Authentication)'.
The process to add AD as an identity source is performed under the vCenter Single Sign-On (SSO) configuration in the vSphere Client.