Here it is. Add this code to your web.config file:
<system.webServer>
<rewrite>
<rules>
<rule name="Root Hit Redirect" stopProcessing="true">
<match url="^$" />
<action type="Redirect" url="/menu_1/MainScreen.aspx" />
</rule>
</rules>
</rewrite>
</system.webServer>
It will do 301 Permanent Redirect (URL will be changed in browser). If you want to have such "redirect" to be invisible (rewrite, internal redirect), then use this rule (the only difference is that "Redirect" has been replaced by "Rewrite"):
<system.webServer>
<rewrite>
<rules>
<rule name="Root Hit Redirect" stopProcessing="true">
<match url="^$" />
<action type="Rewrite" url="/menu_1/MainScreen.aspx" />
</rule>
</rules>
</rewrite>
</system.webServer>