You can do that by doing the following steps:
Step 1: Enable dev console for CS:GO
Step 2: Open console with
~ key and execute this command to start the map:
game_mode 2;game_type 1;exec gamemode_deathmatch; mp_match_end_restart 1; map workshop\149093917\dm_rust;
workshop\149093917\dm_rust is my map, replace it with your map.
To come up with the command, I read the following sources:
CSGO Game Mode Commands.
forcibly execute a deathmatch
How can I prevent automap change?
I want to make a command menu (you know, commandmenu.txt, open by pressing H key) to save me from using console window. But I don't know how to setup command menu in CS:GO. Maybe I'll figure it out later. Currently I just save the command above to a text file and copy paste it to console window.
I figure out how to automatically run the command when the game starts so it saves me from copying pasting the command above to console windows every time I play CS:GO.
Step 1: Create the file
autoexec.cfg by doing the following:
Create a text file (with Notepad), paste the command above to the text file, save the text file as the name
autoexec.cfg, then move this file to the folder
.\Steam\steamapps\common\Counter-Strike Global Offensive\csgo\cfg
Step 2: Set Launch Option for CS:GO by doing the following:
Open Steam, open Properties for CS:GO, in general tab, click
Set Launch Option..., then paste this to the textbox:
-exec autoexec -novid
Note: I read the Launch Options at this page: CSGO : Launch Options