I'm not sure if it suits your needs but you can try this one (and maybe make some small adjustments if the case):
<system.webServer>
<rewrite>
<rewriteMaps>
<rewriteMap name="Register Redirect">
<add key="register.html" value="/register/register.html" />
</rewriteMap>
</rewriteMaps>
<rules>
<rule name="Redirect rule1 for Register Redirect" stopProcessing="true">
<match url="(.*)" />
<conditions>
<add input="{URL}" pattern="^(.*)/register/(.*)(\.html)?$" negate="true" />
</conditions>
<action type="Redirect" url="{HOST}/register/{R:1}" appendQueryString="true" redirectType="Found" />
</rule>
</rules>
</rewrite>
</system.webServer>