So, something like this (could use some syntax help please):

Function Item_Write()
If CheckBox1.Value = True Then
Item.Categories = Item.Categories & ",Business"
End If
End Function

I realize you mentioned I needed this somewhere, but am not sure how to
integrate the code together:

Sub CheckBox1_Click()
Set myinspector = Item.GetInspector
Set myPage1 = myInspector.ModifiedFormPages("Message")
Set Checkbox1 = myPage1.Controls("CheckBox1")
End Sub

Thanks.