type
  TfrmNewMember = class(TForm)
    edtPostcode: TEdit;
  public
    procedure DoSomething;
  end;

implementation

procedure TfrmNewMember.DoSomething;
begin
  edtPostcode.Text := 'TEST';
end;

// then you can call it from the other unit for instance like
var
  NewMemberForm: TfrmNewMember;
begin
  // do not specify Owner if you're freeing the component by yourself; use nil instead
  NewMemberForm := TfrmNewMember.Create(nil);
  try
    NewMemberForm.DoSomething;
    NewMemberForm.ShowModal;
  finally
    NewMemberForm.Free;
  end;
end;