Michel,

Maybe you need to initialize a var in the GoGetValue function, always named the same, and set your return value in the new window to the var instead of trying to put it directly into the form field. I think you can reference the var with

window.opener.varname=whatever

Then, in the GoGetValue function, set the field value to the var value. You could optionally pass in a field reference to the function, which could be called like

GoGetValue(document.MyForm.NoMember)

leaving the function generic. The function then would be:

function GoGetValue(theField)
{
  returnValue = ''; 
  window.open("MyUrl");
  theField.value = returnValue;
}

NOTE: I have not tested this, but in theory it should work. My apologies if the syntax is slightly off...