(sreenshots are from the french STEAM version, sorry for that ...)
1) Launch Steam, and go to <My games> panel
2) Right mouse click on HL2 Deatmatch, and go to <proprieties> item

3) Click on the <launching properties> button

4) Add -console in the input text box

5) Add -novid in the same input box, if you are bored with the intro movie (You can join game server faster without the boring video)