The Tiingo data service is one small step up from Yahoo in terms of quality and reliability. Like Yahoo, they offer a free service level, but limit it even more than Yahoo does, by limiting total symbols per hour to only a few hundred. For a $10/month, this restriction can be removed.
Whether you use the free or $10/month level of service, you must register with them and receive a private API key.
The import_tiingo.rts example script includes comments about how to tell RealTest what your key is in order to use Tiingo data:
As stated in the above comments, you must perform a one-time task of quitting RealTest, editing RealTest.ini, and entering your key in this spot:
Once the INI file is saved, run RealTest and you'll be able to import from Tiingo. Your key will remain preserved in the INI file thereafter unless you delete or replace the file or edit it again and delete the key.