How? Just ask: https://old.reddit.com/r/redditrequest/
I took over some big subreddits this way and I've had big subreddits taken away from me because I forgot to log in and comment on the account I used to create it. Even though I was active on a non-creator/non-mod account. It's actually how I found out about the "feature".
A very poorly implemented feature given the potential value of subreddits and all the work it takes to grow one.
So if you run a subreddit that you care about, remember to assign some active mods even just to avoid someone taking over your subreddit.