I figured out the problem mere seconds after I posted this question.
When I first tried to add the ESXi server I used "Add vCenter" and it was required to enter a domain with the username. When this didnt work I tried to add it as a host, but because I was previously asked to enter a domain I figured this required a domain aswell so I tried various domains.
It turns out when adding a host you dont require a domain, only when adding a vCenter.
So yes, just typing root worked