Redirect all topdomain traffic (without subdomains) to www.

rule.config

<rule name="Redirect all topdomain traffic (without subdomains) to www." stopProcessing="true">
    <match url=".*" />
    <conditions>
        <add input="{HTTP_HOST}" pattern="^[^\.]*\.([^\.]+)$" />
        <!--
            https://regex101.com/r/6be5ka/1/

            ^ asserts position at start of a line
                Match a single character not present in the list below [^\.]*
                    * Quantifier — Matches between zero and unlimited times, as many times as possible, giving back as needed (greedy)
                    \. matches the character . literally (case sensitive)
            \. matches the character . literally (case sensitive)
                1st Capturing Group ([^\.]+)
                    Match a single character not present in the list below [^\.]+
                        + Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
                        \. matches the character . literally (case sensitive)
            $ asserts position at the end of a line
        -->
    </conditions>
    <action type="Redirect" url="https://www.{C:0}/{R:0}" redirectType="Permanent" />
</rule>