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"
       End With
    End If
        'bCancel = < place your custom save criteria here >
    End If

    Set oGift = Nothing
    On Error GoTo 0
    Exit Sub
    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

Disclaimer: We provide programming examples for illustration only, without warranty either expressed or implied, including, but not limited to, the implied warranties of merchantability and/or fitness for a particular purpose. This article assumes you are familiar with Microsoft Visual Basic and the tools used to create and debug procedures. Our Customer Support can help explain the functionality of a particular procedure, but we will not modify, or assist you with modifying, these examples to provide additional functionality. If you are interested in learning more about the VBA and API modules, contact your account manager.