add this code this code under <system.webServer>
section
<rewrite>
<rule name="Redirect to www">
<match url=".*" />
<conditions logicalGrouping="MatchAny">
<add input="{HTTP_HOST}" pattern="^(www\.)(.*)$" negate="true" />
</conditions>
<action type="Redirect" url="http://www.{HTTP_HOST}/{R:0}" redirectType="Permanent"/>
</rule>
</rewrite>