Adding Menu to a Subdomain in Joomla

It's very simple to add a sub-domain in a menu as menu item in Joomla.

From Joomla Administrator Control Panel, select the Menu where you want to add a new menu item.
Then click on Add New Menu Item.
Select the Menu Item Type as External URL found under System Links
Now, provide a Tile in Menu Title field.
Enter the Complete URL of sub-domain in the Link field as below:
eg: http://subdomain.domain.com

Now press the Save button. You are done.