Using Custom Form For Message Box

I use the following code for an error message box to pop up.
I need to know how bring up a custom form I have made instead of the standard message box but don't know how to code it into the function.

Function StartUp()
On Error GoTo Err_ProcedureName

Set db = CurrentDb
Set rs = db.OpenRecordset("tblEmployeeBenefits", dbOpenDynaset)

If rs.EOF = False Then
rs.MoveLast
If rs.Fields("FlagDate") = True Then
MsgBox "You need Administrative access for this function to work.", vbOKOnly, "Serious Warning"
DoCmd.Quit
End If


Else

If rs.BOF = True Then
y = 0

Do Until x = 1
x = rs.RecordCount

rs.AddNew
rs.Fields("MeDate") = Date + y
rs.Update
y = y + 1
Loop
End If
End If
UpdateTable

Exit_ProcedureName:
Exit Function

Err_ProcedureName:
MsgBox Err.Description, vbOKOnly + vbCritical, "Function Start Up"
Resume Exit_ProcedureName

End Function