Automatic correlation and drill down is not possible for custom timers.  Silk Performer Diagnostics integration works by including a custom dynaTrace header in the request to the resource, the Diagnostics server then gets this header and knows to correlate the Silk Performer request to the tagged web request.  However, because MeasureStart/Stop functions do not make network requests there is no opportunity to include the custom dynaTrace header which tells the server to correlate. 

Example:

The following code executes in the test script:

MeasureStart("last minute search");
WebPageSubmit("userlogin", USERLOGIN001, "Choose your trip"); // Form 1
MeasureStop("last minute search");

In the Overview Report the WebPageSubmit function has the integration option enabled:




On the dynaTrace client the "Choose your trip" WebPageSubmit function is visible:





However, the custom timer which is wrapped around the WebPageSubmit function does not have integration capability: