We developed a chess app for iMessage and we would like to have people try it out and tell us their opinions.
Do you know a good strategy to give away free promo codes for the pro version of our new game?
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Go to where your audience is.
You have a chess app, so I would recommend to find some online communities of chess players. Read their community guidelines, FAQs and a bit of their content to make sure that discussing your app is on-topic in the community. Read it carefully: unwelcome advertising is annoying and creates more damage than value. For example, definitely do not do this as a question here or on any other stackexchange site! When you see nothing which hints that talking about your app might be unwelcome, just create a thread where you introduce yourself, ask them if they would like to give you some feedback about your chess app, post a few codes and the promise to give a private code to anyone who contacts you privately.
Also do your research and check if there are any influencers around you could collaborate with. Are there any YouTubers, bloggers, news websites, social media personalities or similar who have a non-negligible reach in the chess community and regularly talk about chess apps? Find a way to contact them directly, give them a promo code and ask if they would like to post about it. The relationship between developers and influencers is beneficial for both sides. They get to report about something which isn't available to the general public yet and you get free promotion. So don't be shy. The worst they can do is ignore you.
Also use the reach of your own social media presence (you do have a social media presence, do you?). Dropping a few codes in a public post will likely burn out quickly. But you could at least announce that you are starting a closed beta and anyone who asks you nicely will get a code. Check if anyone talks about your game on social media. When they have some reach, consider sending them a code, too.
At a very high level, a possible cycle for finding users goes something like this: