An in-play bet is placing a bet when the match has already started and the process does differ from placing a pre-match bet.
We should never do this on Horse Racing markets.
Let's take the Leo Vegas Welcome Offer. We need to place a £10 in-play bet and we should try to do this at half-time in a football match to ensure that the odds are more stable. You cannot use the Oddsmatcher for this process so you will have to find the match manually. You can find out how to do this here.