Redirect Non WWW to WWW using Rewrite module

Using Regex

<rewrite>
    <rules>
        <rule name="Redirect domain.com to www" patternSyntax="ECMAScript" stopProcessing="true">
            <match url=".*" />
            <conditions>
                <add input="{HTTP_HOST}" pattern="^yourdomain.com$" />
            </conditions>
            <action type="Redirect" url="http://www.yourdomain.com/{R:0}" />
        </rule>
    </rules>
</rewrite>

Using Wildcards

<rewrite>
    <rules>
        <rule name="Redirect domain.com to www" patternSyntax="Wildcard" stopProcessing="true">
            <match url="*" />
            <conditions>
                <add input="{HTTP_HOST}" pattern="domain.com" />
            </conditions>
            <action type="Redirect" url="http://www.domain.com/{R:0}" />
        </rule>
    </rules>
</rewrite>