Hello,

Do you mean you finished all the setup procedures, and then try to connect to the extender's wifi, then it ask for a wifi password? What's the extender's wifi name? If you set up the extender's wifi successfully, usually the default wifi name will disappear, and it copied the router's wifi name and password. 

Please factory reset the extender by press the reset button on it for 8 second, until the light change, then refer to the manual below to reinstall it. Please change the extender's wifi name to another one in step 5, then you can tell which wifi is from the extender, and which one is from the router. Finally connect to the extender's wifi and test the internet, password is the same with your router.

https://www.tp-link.com/en/faq-930.html