This module will provide simple Login and Register link as a block plugin, which will load the Drupal Login form and Register form in a popup..
Download and activate the module
Under Structure => Block lay-out remove the original Drupal Login block.
Place one of the three provided with the module in a block region You can choose for only login form, only register form or a combinatoir of the two.
The login and register links will only be visible when you are not logged in.
Klick on the login link and the pop-up wil appear.