You can collect opt-ins and send Web Push notifications if you have multiple domains.
Can multiple domains show only one opt-in window and use the same API for the same Web Push notification?
You can define multiple domains under a single panel, like domainA.com and domainB.com. If a user opts in for one of them, they will not see the opt-in permission screen again in other domains, and it will be counted as an opt-in for different domains.
If you want to send different pushes for different domains, you can define different languages for each. For example, when sending a Single Web Push for all domains, you can choose the "All Languages" option in the Segments step so that all domains will receive the Web Push notification.
How will opt-ins be collected with native opt-in integration?
You might have two subdomains, such as www.mywebsite.com & store.mywebsite.com, and the same Insider API integration is applied to both domains. For native opt-in, Insider's Service Worker (insider-sw-sdk.js) must be accessible both from www.mywebsite.com/insider-sw-sdk.js and store.mywebsite.com/insider-sw-sdk.js
Let's say Andrea saw the opt-in prompt while visiting the store.mywebsite.com domain and allowed it. When he visits www.mywebsite.com, Insider won't display the opt-in again since he already subscribed to the main mywebsite.com domain. Norah saw the opt-in prompt while visiting www.mywebsite.com domain and allowed it. Similarly, when she visits store.mywebsite.com, Insider won't display the opt-in again since she already subscribed to the main mywebsite.com domain.
When you send push notifications from the panel, Andrea and Norah will receive the same push; the only difference is that while John will see it coming from store.mywebsite.com, Norah will see it from www.mywebsite.com.