Is there a website where you can play against real people with an opening already installed by default? For example, I want to play against the English opening, but my opponents rarely play it, so I want to practice somewhere.
If you want, Chess.com allows you to play from a custom opening position against a computer, which you can choose your level. If you need help with it, go here. You can also send a game to a friend.
Lichess does a similar thing. Create your personal board, and press continue from here to play with a computer or a friend.
Lichess also has several thematic tournaments throughout the week. As an example take a look at this thematic Italian game arena tournament:
When you open the tournament, the opening is mentioned in the upper left corner if there is one:
It is a link so you can click it to see the mentioned opening.
Alternatively you can create your own tournament with a opening of your choice. For that just go to the create tournament area and set the starting position:
In that box you have to place a FEN string. For the English opening considering only
1 C4, you can use the following fen:
rnbqkbnr/pppppppp/8/8/2P5/8/PP1PPPPP/RNBQKBNR b KQkq - 0 1
There are theme tournaments on ICCF. Example for the Alapin variation of the Sicilian. However, it is correspondence chess, which might not be your cup of tea.
echecsemail.com allows you to create games and tournaments dedicated to specific openings.