While iOS 11.3 does support web app manifests, it does not yet support specifying icons this way. You’ll want to include it in your manifest for other devices, but at least for now you’ll have to use the following for iOS:
<link rel="apple-touch-icon" sizes="180x180" href="https://stackoverflow.com/questions/49568333/icon.png">
Specify the icon size, and include a URL.
Learn more at Apple’s documentation here
There is also a website to automate the process linked here