What is the code or data source for your Add Product button currently?
You probably just need to change that to your IsActive query.

Screenshots?

Private Sub AddBtn_Click()

    If IsNull(ProductCombo) Then
        DoCmd.GoToControl "ProductCombo"
        ProductCombo.Dropdown
        Exit Sub
    End If
    
    If IsNull(Forms!OrderF!OrderID) Then
        Forms!OrderF!OrderDate = Date
        Forms!OrderF.Refresh
    End If
    
    ' check for duplicate product on order
    Dim C As Long
    C = DCount("*", "OrderDetailT", "OrderID=" & OrderID & " AND ProductID=" & ProductCombo)
    If C > 0 Then
        ' product already exists on this order
        If MsgBox("WARNING! This product is already on this order. Do you want to add it again?", _
            vbYesNoCancel + vbExclamation) <> vbYes Then Exit Sub
    End If
    
    DoCmd.GoToRecord , , acNewRec
    DoCmd.GoToControl "Quantity"
    ProductID = ProductCombo
    ItemNumber = ProductCombo.Column(1)
    UM = ProductCombo.Column(2)
    ProductDescription = ProductCombo.Column(3)
    UnitPrice = ProductCombo.Column(4)
    Notes = Nz(DLookup("Notes", "ProductMasterT", "ProductID=" & ProductCombo), "")
    Me.Refresh
    
End Sub

Add a Where condition to your Combo Box so you don't see inactive products.

WHERE IsActive=True

For the rowsource of your combo box add the IsActive = True there.

It will look something like this:

SELECT ProductID, Description, UnitPrice, IsActive
FROM ProductT
WHERE IsActive =True
ORDER BY Description, UnitPrice;

This assumes IsActive is defaulted to True.

Thank you so much.  I knew it was a simple where condition but, I just could not figure out the placement.