1. iOS universal links:
1.1. Put apple-app-site-association to FBA IIS site root folder
1.2. Add the following section under <configuration> section in web.config (together with other <location> elements):
 <location path="apple-app-site-association">
    <system.web>
      <authorization>
        <allow users="*" />
      </authorization>
    </system.web>
    <system.webServer>
      <staticContent>
        <mimeMap fileExtension="." mimeType="application/json" />
      </staticContent>
    </system.webServer>
  </location>

2. Android app links:
2.1. Create .well-known sub folder in FBA IIS site's root folder
2.2. Put assetlinks.json to created .well-known sub folder
2.3. Go to IIS manager > FBA IIS site and convert .well-known sub folder to Application
2.4. Put the following web.config to .well-known sub folder:
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.web>
      <authorization>
        <allow users="*" />
      </authorization>
    </system.web>
    <system.webServer>
      <staticContent>
        <clear />
        <mimeMap fileExtension=".json" mimeType="application/json" />
      </staticContent>
        <handlers>
            <remove name="JSONHandlerFactory" />
        </handlers>
    </system.webServer>
</configuration>