Joining Existing Windows Active Directory Domain

First timer here, any help is appreciated.

I am attempting to join the SuiteCRM server at configuration time to my existing windows environment as a member server.

In the attached screenshot that server it’s pointed to is a standard Server 2012 R2 DC. That account is a domain admin level account. Is there a step I’m missing here? Does the username need to be formatted with a domain\ prefix or an suffix? Are there prerequisites on the Windows DC that need to be performed?

Any help is appreciated?