It’s super easy one way, and not so easy but doable another way.
Super easy – just use IFTTT.
Use Alexa channel as the IF and SmartThings Channel as the THAT in an IFTTT recipe and you’re done.
“Alexa, trigger unlock the front door” or whatever phrase you want to use after the word “trigger.”
Use a virtual switch.
2a) create a routine that unlocks the door.
2b) create a virtual switch.
2c) tie that virtual switch to the routine. (Alexa Helper is one way to do this)
2d) authorize the virtual switch to Echo.
From then on “Alexa, turn on myswitch” will turn on the virtual switch which will run the routine which will unlock the door.
There are links in the Echo integration FAQ, and yet another method.