(I'm on the app also, I don't know if it works on desktop too).
Go to futures trade tab (fourth across), select the type of futures, three dots (options) and click "preferences"
Click on "position mode settings" and press to switch it to "hedged mode".
Now go back and you'll see a window for opening new positions and one for closing old ones (you might want to familiarise yourself with how these work).
Make sure you also put the mode into cross margin instead of isolated too so you can use your entire balance.
Margin is provided to me as percentages, so you'll just want to do your 100/leverage to work out percentages. So if you have a 20x long, 100/20 is 5% of your whole funds. (subtract a percent from this to allow for the maintenance margin and half it for leverages over ~30x just in case).
For the counterposition, if using your whole funds, you'll then want to work out how many % up you'll be if in profit and adjust your % allocation accordingly. I wouldn't recommend putting 100% of everything on a trade, if you're playing with up to 5% of you're portfolio though and aren't susceptible to obtaining an addiction then it's probably fine.