@Lucky2662 

Do you mean you use the same credentials  to log into the Tether app? 

For your information, you need use the cloud account to login to the Tether app; then once you logged in, you will see the device and then you can click it and enter the same credentials for actual websites to login. 

Below is the cloud website where you can register the cloud account.

https://www.tplinkcloud.com/register.php

May it help.