I have tested and it appears that the red marking of the contacts in the list is a bit hit and miss, however the following sets the flag the same as the dialog. so it may work better for you. If not I regret I don't know how to ensure that the red emphasis is displayed.

Public Sub SetReminder(dHours As Double, _                       Optional bMin As Boolean, _
                       Optional lngDue As Long = 0)


'Set lngDue as follows from the calling macro. The default is 0
'0 = olMarkToday
'1 = olMarkTomorrow
'2 = olMarkThisWeek


Dim olItem As Object
Dim dTime As Date
    Select Case TypeName(Application.ActiveWindow)
        Case "Explorer"
            Set olItem = ActiveExplorer.Selection.Item(1)
        Case "Inspector"
            Set olItem = ActiveInspector.currentItem
    End Select


    If TypeName(olItem) = "ContactItem" Then
        dTime = CDate(Now)
        If bMin = True Then
            dTime = DateAdd("n", dHours, dTime)
        Else
            dTime = DateAdd("h", dHours, dTime)
        End If
        With olItem
            .MarkAsTask lngDue
            .ReminderSet = True
            .ReminderTime = dTime
            .Save
        End With
    End If
    Set olItem = Nothing
End Sub


Sub AddTwoMinutes()
    SetReminder 2, True, 0
End Sub