See the FAQ (this is a clickable link). The short answer is this cannot be done at the present time except by setting up a separate speaker device to speak the command that you want so echo can hear it.
Breaking News!!! As of August 2018, a SmartThings-connected Contact or Motion Sensor can now trigger an Amazon echo routine if you live in the US, Canada, or the UK !!!
Amazon has now added the ability for some sensors to trigger an Amazon echo routine (not a smartthings routine), which does let you trigger other smart home devices which are not connected to SmartThings without having to speak an echo command! Or you can set it up so that your echo devices will speak a customized phrase, play a doorbell chime, or various other sound effects.