The only way my colleagues have found to do this is via appcmd. If you are on ansible 2.2. you could use win_command instead of raw but the rest of the syntax would probably be the same as below:
- name: Add authentication for Site
raw: 'C:\Windows\System32\inetsrv\appcmd.exe set config "Site/" -section:system.webServer/security/authentication/anonymousAuthentication /enabled:"True" /commit:apphost'
- name: Add authentication user for Site
raw: 'C:\Windows\System32\inetsrv\appcmd.exe set config "Site/" -section:system.webServer/security/authentication/anonymousAuthentication /userName:{{ iis_domain }}\{{ iis_user }} /password:{{ iis_cred }} /commit:apphost'
HTH
Jon