I think you can select the opening that you want to play to be set up automatically in your challenges. I don't use that feature because I need to be able to adapt to a multiple number of responses in real life OTB. I can't play the lines I like all the time.
Pre-Game Conditionals

I've seen seeks where the opening position was already set, but I don't know how to do it.
Go to 'more advanced options' in your challenge.

I'm aware of this feature where you can set the opening in the seek. I think people are less likely to select seeks where the opening is set unless they're specifically interested in playing that as well. A normal seek allows you to have the openness that atomichicken mentioned, the opponent can play whatever they want, but if there are several moves that I know I will play with, why not set those up as a conditional in your settings. This allows more flexibility.

I'm aware of this feature where you can set the opening in the seek. I think people are less likely to select seeks where the opening is set unless they're specifically interested in playing that as well. A normal seek allows you to have the openness that atomichicken mentioned, the opponent can play whatever they want, but if there are several moves that I know I will play with, why not set those up as a conditional in your settings. This allows more flexibility.
Oh, I see now you're not talking about setting a pre-arranged opening, but a series of conditionals right from the start in your challenge (your opponent shouldn't know about them obviously). It sounds like a good idea.

I'm aware of this feature where you can set the opening in the seek. I think people are less likely to select seeks where the opening is set unless they're specifically interested in playing that as well. A normal seek allows you to have the openness that atomichicken mentioned, the opponent can play whatever they want, but if there are several moves that I know I will play with, why not set those up as a conditional in your settings. This allows more flexibility.
Oh, I see now you're not talking about setting a pre-arranged opening, but a series of conditionals right from the start in your challenge (your opponent shouldn't know about them obviously). It sounds like a good idea.
Right. Just as in OTB you can't choose the opening entireley, but you do know how you're going to respond to e4, d4, c4, and so on, so why not have the option here to speed it up a bit. It could be set up per seek, but even that would be a bit annoying. It would be better in the personal account options like board size and stuff. That way I can decide, okay for one month I'll always play c6 to e4, d5 to d4, e5 to c4, for example. I don't know if this is how other people play but I usually pick a few responses and stick with them for a month or more so I get lots of practice with it and then decide to try something else or if I really like it it becomes my standard response.
Lately I've been wanting to practice the Caro-Kann defense, so every game I start as black I answer 1.e4 with c6. It would be nice to be able to go into my settings and set it up so in all my new games, e4 is met with c6 automatically. Then d4 would be met with d5, etc. This way I can jump into my games once we're through the rehearsed part.