Use the RE Code Wizard and modify the resulting code to suit your needs. It should be placed under RE > Tools > Visual Basic for Applications > System > RE_System_Object Code. Below is an example of changing the post status of a gift to "Do Not Post".
'This procedure will be called before a record of the specified type is saved
Public Sub Gift_BeforeSave(oRecord As Object, bCancel As Boolean)
    'oRecord              : record object being saved
    'bCancel              : set to true to cancel the save operation
 
    Dim oGift As CGift 'Current open gift record in read only mode
       
    On Error GoTo ErrHandler
   
    Set oGift = oRecord
     
    If Not oGift Is Nothing Then
     If oGift.Fields(GIFT_fld_Fund) = "2003 Membership Fund" Then
      With oGift
           .Fields(GIFT_fld_Post_Status) = "Do Not Post"
           .Fields(GIFT_fld_Post_Date) = "02/26/2016"
           .Save
       End With
    End If
        'bCancel = < place your custom save criteria here >
    End If

    Set oGift = Nothing
   
    On Error GoTo 0
   
    Exit Sub
   
ErrHandler:
    Dim sErr As String
    sErr = Err.Description
    On Error GoTo 0
    '< place your custom error handling code here >
    MsgBox "Error processing Gift_BeforeSave : " & sErr
   
    Set oGift = Nothing
    bCancel = False
   
    Exit Sub
 
End Sub

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