Can't figure out the rule to add "www" to domain

our web application has many domains pointing to it and I am trying to "correct" incoming requests that lack the preceding "www" from the domain name

I have this rule in effect and it works fantastic, I just cannot figure out the syntax to prepend the "www" to *any* domain name that comes it with just two parts

Anyone able to lend a hand with the syntax?

<rule name="Add WWW prefix - Client A" stopProcessing="true"><match url="(.*)" ignoreCase="true" /><conditions><add input="{HTTP_HOST}" pattern="^client-a-domain\.com$" /></conditions><action type="Redirect" url="http://www.client-a-domain.com/{R:0}" redirectType="Permanent" /></rule>

so pretty much a rule like "if a domain name comes in that is only two parts, rewrite it so it's three parts

thanks in advance !