Im not sure if i understand your request, from what i understand, you may either want to add yahoo as the default search engine, or open yahoo as the home/new tab.. The default engine can be changed in options -> sidebar "search" -> select the search engine. To set yahoo as the default/main page you go to options -> sidebar "Home" -> Homepage and new windows / New tabs, select the drop down "custom URLs..." Write the URL.

To set it into a tile, you just click add shortcut