Use the following command to convert the web application to use claims-based authentication:
$App=Get-SPWebApplication [-Identity <the URL of the Web application>]
$App.UseClaimsAuthentication = "True"
$App.Update()
For example:
$App=Get-SPWebApplication "http://fba.companyabc.com"
$App.UseClaimsAuthentication = "True"
$App.Update()
Note
Converting the web
application to claims-based authentication cannot be reversed. Make sure
that the web application is ready to be converted.