PowerBi Report Builder (Paginated Reports) - parameter connected to different dataset than report

Hello,

I'm creating a report using two datasets. One dataset is used for a parameter where the user chooses a specific username, and the other dataset is used for the rest of the report. How do I create a relationship between these two datasets so that the username parameter will filter the whole report? I know you can use a lookup function in a table to grab data from another dataset, but how do I do this with a parameter? Thank you in advance!