When you use the social share addon, you're using a Facebook app built by Playable.
If you use a custom domain instead of your Playable subdomain, you have to set up your own app before you can use the social share addon. Without doing this, it won't be possible for participants to share their results on Facebook.
See in the video how to set up a Facebook app. The steps are written below.
Go to https://developers.facebook.com/ and add a new app at the top of the screen.
Enter the name of your app. If you only have one custom domain with Playable, you can give your app a generic name. If you have more than one, give it a name that will help you identify it.
Confirm your contact email. You can add additional administrators later under "roles" in your Facebook developer account.
The start page has a list of products. You don't need to select one — just click on "settings" in the side menu and select basic settings.
Here you will enter the following required information:
App domain (the custom domain you have set up in Playable)
Privacy Policy URL and Terms of Service URL (links to these on your website; it may be the same URL)
Category (the category that best describes your business, not your Playable campaign)
At the bottom of the page, click to add a platform and choose website. Here you will enter your custom domain again, this time the full URL, including https://
Click save, and at the top of the page, you can activate your app, moving the status from "in development" to "live".
Copy the app ID to your Playable account's global settings.
If you have multiple custom domains, you will need to create multiple apps. Instead of entering the app ID in your global settings, you will enter it in the individual campaign, based on which domain it uses.