At the start of the expansion you will find yourself in the Resistance Base. After listening to the first guy that greets Brady (I always called him Conor McGregor) you will end up walking into the subway platform room and to your immediate right there will be a guy at a table funneling ingredients inside of a beer bottle. He will ask you to help him make a few Molotov and you will have the 2 ingredients sitting on the table. After making 3, you can deposit 2 of them by looking at the table and pressing "X" twice.
This is the most important part. This has to be the first thing you do because you wont be able to deposit the molotov once the door opens for you to see Parrish. The good thing is that you don't have to break any records since the game gives you a nice amount of time...I just happened to miss the 2 huge ingredients on the table so I was looking around the area for a bit, saw them on the table and after I made them I no longer had the option to deposit them.
So as a recap, once you load up the DLC you will need to quickly run up to the table, grab the 2 ingredients and make at least 2 Molotov as the guy is asking you for help and then look at the table and press "X" to deposit them. If you happened to be a bit slow and you hear someone say "Parrish is ready to see you" just restart from the last checkpoint and try it again.