determining the address of the server?

The way my app has been designed is that each server has a copy of wordpress, copy of the application, and a copy of the fusionAuth server. When a user reaches my website, researchStudyOnline.com, the AWS load balancer determines which server will be handling this request by displaying my landing page. My landing page has a button: Login to Research Study Online. This will bring them to my app at a certain URL on that server. 

On my laptop "server" the address of the server is localhost. So to login they get sent to localhost/index.php/Login.
Bur how will this work on my AWS server? Will localhost still be the local server or will I use xx.xx.xx.xx/index.php/Login?

proof that an old dog can learn new tricks