Can anybody help me please.

I have an outlook custom form and I would like to disable certain fields based on an option button. I have 2 option buttons on my form but how do I refer or change the properties at runtime when the user selects the option button:

I suppose I need to some kind of procedure that refers to the control like below

Item.GetInspector.ModifiedFormPages("Message").Controls("TextBox1").Enabled = True
Item.GetInspector.ModifiedFormPages("Message").Controls("TextBox1").Readonly = True

Many tHanks