Forms :: Manage MsgBox With Blank Required Fields By Function In A Module

I created this function to manage a MsgBox containing all required fields with no data:

Code:
Function FormValidation(TheForm As Form) As Boolean
Dim boolResponse As Boolean
Dim strError As Variant
Dim ctl As Control
boolResponse = True
strError = Null

[code]...

Then, I have a Form_BeforeUpdate event, where I wanna place the function, which contains also some VBA code to manage duplicates records:

Code:
Private Sub Form_BeforeUpdate(Cancel As Integer)
Dim rst As DAO.Recordset, dbs As DAO.Database, strICAO As String
Dim ctl As Control, txtMsg As String

[code]....

Now, how to add the function in this event to get these two results:

1. if required fields are blank and I press OK on the MsgBox, the routine must stop;
2. the focus must go in the first required blank control.

I tried some options but I get different kind of malfunctions: no custom message for duplicate records but only the access default one, "go to next record" feature with tab key not working and so on.