Click on Manage researchers. If you have already defined researchers before, you can choose these from the dropdown menu under Choose researcher. To define a new researcher, click on Create new researcher. 
Give their name and email address.
Click on create. If you want to, you can also provide their ORCID (https://orcid.org). This helps to distinguish any researcher from every other researcher.
You will not be able to publish any metadata without providing at least the name and email address of one researcher.