Nginx proxy on Shiny server

[nicoperrot]

I have a Shiny App running on a server that I can access using IP:3838/testapp/.
I would like this link to not be like that, but rather have a real link like http:shinytestapp (something like that), so I set up a nginx proxy using the following code :

map $http_upgrade $connection_upgrade {
       default upgrade;
       ''      close;
     }

    server {
	listen       80;
        listen       [::]:80;
        server_name  _;
        root         /usr/share/nginx/html;

        # Load configuration files for the default server block.
        include /etc/nginx/default.d/*.conf;

        error_page 404 /404.html;
        location = /404.html {
        }

	error_page 500 502 503 504 /50x.html;
        location = /50x.html {
        }

	rewrite ^/shiny$ $scheme://$http_host/shiny/ permanent;
        location /shiny/ {
          rewrite ^/shiny/(.*)$ /$1 break;
          proxy_pass http://localhost:3838;
          proxy_redirect / $scheme://$http_host/shiny/;
          proxy_http_version 1.1;
          proxy_set_header Upgrade $http_upgrade;
          proxy_set_header Connection $connection_upgrade;
          proxy_read_timeout 20d;
          proxy_buffering off;
    }

How can I now access my app using a real link? This is the confusing step for me.
Thank you!