As far as currently documented supporting app indexing under iOS requires 2 steps:
Support HTTP deep links in your mobile app as per Apple's documentation
This involves creating the apple-app-site-association
file that contains JSON data about the URLs that your app can handle, uploading the apple-app-site-association
file to your HTTPS web server (to create a trust relationship between your site and the app) and adopting the UIApplicationDelegate methods.
Integrate the App Indexing SDK as per Google's documenation
Install the GoogleAppIndexing
pod and register your app in your application:didFinishLaunchingWithOptions:
as follows
[[GSDAppIndexing sharedInstance] registerApp:*your iTunes ID*];
Additional notes
It's important to remark that there must be a certain degree of coherence between the web and app content as stated in the Android documentation
You can enable Google to crawl through your app content and present
your Android app as a destination to users through Google Search
results, when that content corresponds to a web page that you own.
If the crawler determines there's a significant difference between the 2 it might complain and give you a Content mismatch error in the search console. See the crawl error FAQ.
Keep in mind that the app indexing seems to be still in a preliminary phase under iOS given that some information are available only for the Android platform, so ymmv.
Further reference
Check the WWDC video Seamless Linking to Your App for a full implementation example. They talk about entitlements @ 23:20.