Make a start file

In the root folder of your extracted build, create a file called "start.bat".

Open it in Notepad, and depending on your preferred window settings, copy and paste these into it, and run the batch file.

Normal start.bat

hl2.exe -game portal2 -tempcontent

Borderless window start.bat

hl2.exe -game portal2 -tempcontent -window -noborder

Windowed start.bat

hl2.exe -game portal2 -tempcontent -window

Linux users

Linux builds suck, just use Proton for this.

Personally I would add hl2.exe to Steam as a Non-Steam Game (tutorial), force your preferred Proton in Compatibility settings, then add the launch option

-game portal2 -tempcontent