What is the right xdt:Locator parameter to transform this node?

Ok. I feel a little silly but the solution is that I didn’t need to specify a xdt:Locator.

If I just leave the App.Release.Config like this it will replace matching entry.

 <parameter value="data source=dbserver;Integrated Security=SSPI;Initial Catalog=someDb;MultipleActiveResultSets=true" 
               xdt:Transform="Replace"/>
  </parameters>