I got it

this is the sample code:

<rule name="remove htm extensions">
        <!--Removes the .htm extension for all pages.-->
        <match url="(.*)" />
        <conditions logicalGrouping="MatchAll">
            <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
            <add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
            <add input="{REQUEST_FILENAME}" negate="true" pattern="(.*).htm" />
        </conditions>
        <action type="Rewrite" url="{R:1}.htm" />
    </rule>