This post is specifically for browsers that use the Chromium engine (Chrome [duh], but also anything on this list) but most browsers have a similar feature, you'll just need to dig for it.
Setting a custom search keystroke is helpful because you can target your results without any hassle. For instance, when I type "dr" into my search bar before my terms, my browser only returns items in my Google Drive. It makes my seaching faster because I don't have to open a site first.
In Chrome, go to your Settings. (A quick way to get there is to type
chrome:settings into the address bar.)
In Settings, find Manage Search Engines. This is where you can specify some custom places to look.
Each search engine has to be configured with two pieces: the search url and the search term. Depending on the site, this can be obvious or really unclear. The best method I've found is to go to the site, do a search, and then do some copy/pasting.
Let's say you want a quick search for YouTube. If you open YouTube, do a search for something. We'll use dogs because why not. When you seach, you get this URL:
We're going to replace dogs with a special placeholder: %s. This tells the browser to substitute that term with what you typed.
Click on Manage Search Engines and then click on Add. In the pop up, type in the name of your search engine and then a custom key (or keys) to trigger that search. Then, paste in your search address (see above).
Save the search term with Add. If you open a new tab and type
yt, you'll see a prompt to search YouTube by pressing Tab.
Helpful Search URLs
Here are some of my most used searches so you don't have to go and make your own: