If you're using the solution from the "Composing WinForms with UserControls" solution, that one lets the TabPage handle construction of the data retrieval query. It then passes that query to UserControl's LoadMainData() method.
Here, for example, is the LoadData() method from the EmployeeTabPage. Note the highlighted line:
public override void LoadData() {
_orderUserControl.LoadSupportingData();
_employeeUserControl.LoadSupportingData();
EntityQuery<Employee> anEntityQuery = _entityManager.Employees;
_employeeUserControl.LoadMainData(anEntityQuery);
}