- txtcompanyname.seltext = ucase(txtcompanyname.Seltext)
This statement should do the trick.
But note that as with any other statement, it only does its job once, when you execute it. So if it is done in the Form_Load event, it will happen once while the form is being loaded. If there's nothing in the textbox at that time, it won't have any effect.
Hm... on further consideration, I doubt this will ever have any effect. Because you are referring to SelText, which is presumably the selected part of the text. How could you have selected any of the text before the form is even displayed?