url - Show template in Joomla (parameter) -

I know it's possible in Joomla! To temporarily set the template to another template with a URL parameter. What is the parameter? And how should the template be pointed out? Me

  index.php? Template = mytemplate  

if you go to template manager and on one Click and then edit, then there is a preview button where you can see it in any form in the browser. There is also a link that says "Open in a new window". When I click it, my site is "tp = 1 & amp; The template = ja_purity "

loads as a query,