Nginx reverse proxy to add CORS support to an API. Tested and working with Firefox 68.9 and Chrome 80.0

nginxconfig.ini

server {
    server_name     xxx

    location / {

        # Activer le proxy
        proxy_set_header                X-Real-IP $remote_addr;
        proxy_set_header                X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_pass                      https://yourapi;
        proxy_redirect                  off;
        proxy_buffers                   32 16k;
        proxy_busy_buffers_size         64k;

        add_header    'Access-Control-Allow-Origin' '*' always;
        add_header    'Access-Control-Allow-Methods' 'GET, POST, OPTIONS' always;
        add_header    'Access-Control-Allow-Headers' 'User-Agent, Origin, X-Requested-With, Content-Type, Accept' always;
        add_header    'Access-Control-Allow-Credentials' 'true' always;

 	if ($request_method = OPTIONS ) {
          add_header    'Access-Control-Allow-Origin' '*' always;
          add_header    'Access-Control-Allow-Methods' 'GET, POST, OPTIONS' always;
          add_header    'Access-Control-Allow-Headers' 'User-Agent, Origin, X-Requested-With, Content-Type, Accept' always;
          add_header    'Access-Control-Allow-Credentials' 'true' always;
          add_header Content-Length 0;
          add_header Content-Type text/plain;
          return 200;
	}
    }
}