How i download my Steam games and their updates/DLC is that i queue them up on my laptop which download them to Portable HDDs and then when i want them on my gaming PC i copy the games from the Portable HDDs (i am slowly building scripts to automate the process), generally i buy the games and DLC using the browser either on my Laptop or on a different computer all together (like at work)
When Borderlands 2: Commander Lilith and the Fight for Sanctuary was released for free i went to get it since i had all the other Borderlands 2 DLC and wanted to get it ready for when i did get to play Borderlands 2 because at the time my new PC wasn't ready and my PC at the time could not play it.
As i have never been able to add Free DLC to the cart via the browser i booted up the client and added got it via that, however after it was downloaded Borderland 2 tried to boot up and on my laptop it struggled but i was able to terminate the process.
I'm not sure if this was a once off with Borderlands 2 or not but i would like be able to get Free DLC similar to how i get Paid DLC by adding it to the cart and then it either triggers an "update" to then download or it'll be downloaded when i get around to downloading the base game.
So is there a way to add Free DLC so that it's queued and downloaded but not start the game after?
EDIT: while this question is almost the same, the issue is that i already do that for Paid DLC and with Free DLC, unlike what the answer shows does not register,
Hovering over the first button shows a Steam App URL which wont work on a system that doesn't have Steam installed and the second button goes to this about page
Also the URL hiving over the download button is different to what is presented in the answer