The crucial point in open relationship is being honest.
It is not always easy, as everything in need of discussion is considered personal and therefore nobody is willing to raise this subject too often. But lack of transparency about your preferences might bring your date into a confused state. Some happen to be strongly monogamous, others have misconceptions about what being in open relationship is like. But whatever the reason is for a partner-to-be, the responsibility of a poly person is to make sure everybody is aware of what’s going on.
You shouldn’t assume that the person you’ve never asked about the polyamorous relationships will accept the idea or totally refuse it. Nowadays it’s pretty difficult to predict other people’s reaction to anything. so being honest and straightforward is your strongest weapon as well as your best tool. More than that, remember that the way you choose to give the information can affect a person’s opinion a lot.
Best time to start is while creating a profile on a dating app or during the first conversation if you met your crush offline. If being upfront with the fact of polyamory seems too daring, the best way of warning your date is bringing the subject up in the conversation as soon as possible. This might turn away certain amount of people, but when you end up on a date with somebody, you will be totally sure open relationship is accepted by your partner. Also, nobody will feel sorry or angry if this fact comes out later on a serious stage already. So honesty is the best way to save time and nerves, both yours and your alleged date.
Obviously, don't forget that words like “do what you want” or “I don’t care” don’t mean your partner approves your idea of finding another partner for a polyamorous relationship. Monitor your partner’s reaction and emotions to understand their real attitude completely in advance, or else you’ll risk to lose them.