hi @mssa12345 ,
you can do so like this:
Dropdown Control for Office:
Distinct(ListB, Office)
Button or OnChange Event to Populate Team Lead:
Set(varTeamLeadEmails, Concat(Filter(ListB, Office = DropdownOffice.Selected.Value), TeamLead.Email, ", "))
DefaultSelectedItems Property of the Team Lead Field in the Form:
Filter(
Office365Users.SearchUser({searchTerm: varTeamLeadEmails}),
DisplayName in Concat(Filter(ListB, Office = DropdownOffice.Selected.Value), TeamLead.DisplayName, ", ")
)