You cannot use that syntax in the code of the same form that you're using to open form B. You must use
Forms!myFormNameHere.theControlNameHere.Enabled = false
However, if you're going to do this regardless, as you have shown, why not just set that property in form design and be done with it? Are you setting focus (going to) some control because cboKunde has the focus when you open the form? If so, could just either remove it from the tab order or set the tab index to something other than zero.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.