Macro starts on clicking only certain column cells

Firstly you need to move the macro to a Standard Module. Then add this code to your worksheet event, it will limit the action to the specified Columns & only empty cells

Option Explicit


Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If Target.Column < 5 Or Target.Column > 9 Then Exit Sub
    If Not IsEmpty(Target) Then Exit Sub
    AddToCell
End Sub

Insert a Module in the VBA from yhe VB EDitor Insert menu. Cut the AddtoCell macro & paste it into the new module. Then overwrite the existing code in the sheet module with the new.

Option Explicit


Private Sub Worksheet_SelectionChange(ByVal Target As Range)
'limit code to Columns E to I
    If Target.Column < 5 Or Target.Column > 9 Then Exit Sub
'Only work if cell has a value, you probably don't need this.
    If Not IsEmpty(Target) Then Exit Sub
'This might be better for you.
'It will limit the code to run on cells in the specified Columns that contain a Numeric Value
 If Not IsNumeric(Target) Then Exit Sub
'Calls your Macro
    AddToCell
End Sub

In the notes I suggested that you might want to remove this line

'Only work if cell has a value, you probably don't need this.
    If Not IsEmpty(Target) Then Exit Sub

Like this:

Option Explicit


Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If Target.Column < 5 Or Target.Column > 9 Then Exit Sub
'This might be better for you.
'It will limit the code to run on cells in the specified Columns that contain a Numeric Value
    If Not IsNumeric(Target) Then Exit Sub
    AddToCell
End Sub