Your config could look like this:
server {
listen 80 default_server;
listen [::]:80 ipv6only=on default_server;
server_name pretalx.mydomain.com;
}
server {
listen 443 default_server;
listen [::]:443 ipv6only=on default_server;
server_name pretalx.mydomain.com;
ssl on;
ssl_certificate /path/to/cert.chain.pem;
ssl_certificate_key /path/to/key.pem;
location / {
proxy_pass http://localhost:8355/;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto https;
proxy_set_header Host $http_host;
}
}