It's doable of course.
But as often, it depends on a lot of factors.
In what you explain you don't mention if you'd expect the player to be able to draw more cards from the deck, or if you'd expect the drawn card to be "removed" from the actual deck.
So in answer to your current question you want your keys to be named like "Card_1", "Card_2", "Card_3" etc...
Something like that which is using a number.
Consider also you are having a dropdown list plugin named "List"
Then, in an "On start of layout" event, you make a subevent "Repeat 5 times".
As it action you use the "Add item" action of the object "List" and set it as ("Card_" & int(random(MAX_NB)))
Where MAX_NB is the greatest number in your dictionary cards + 1.
This simple event allows you to display on start of layout five random cards taken from dictionary keys.
To be able to fiddle more precisely and remove cards from the deck so there are no duplicates picked, I'd advised you to look into the Array plugin and possibly this tutorial that helps with making/shuffling and drawing from a deck.