nginx proxy_pass add a static parameter

nginx.conf

worker_processes  1;
events {
    worker_connections  1024;
}


http {
    include       mime.types;
    default_type  application/octet-stream;

    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  logs/access.log  main;

    sendfile        on;
    keepalive_timeout  65;
    
    server {
        listen       47777;
        server_name  localhost;

        access_log  logs/47777.access.log  main;
        
        set $token ""; # declar token is ""(empty str) for original request without args,because $is_args concat any var will be `?`

        if ($is_args) { # if the request has args update token to "&"
            set $token "&";
        }

        location /test {
            set $args "${args}${token}k1=v1&k2=v2"; # update original append custom params with $token
            # if no args $is_args is empty str,else it's "?"
            # http is scheme
            # service is upstream server
            proxy_pass http://127.0.0.1:46666$uri$is_args$args; # proxy pass
        }
    }
    
    server {
        listen       46666;
        server_name  localhost;

        access_log  logs/46666.access.log  main;
        
        location / {
            root   html;
            index  index.html index.htm;
        }
    }

}

http://localhost:47777/test ==> http://127.0.0.1:46666/test?k1=v1&k2=v2
http://localhost:47777/test/ ==> http://127.0.0.1:46666/test/?k1=v1&k2=v2
http://localhost:47777/test/?a=a&b=b ==> http://127.0.0.1:46666/test/?a=a&b=b&k1=v1&k2=v2
http://localhost:47777/test?a=a&b=b ==> http://127.0.0.1:46666/test?a=a&b=b&k1=v1&k2=v2

the access_log

127.0.0.1 - - [14/Dec/2017:12:01:03 +0800] "GET /test?k1=v1&k2=v2 HTTP/1.0" 404 571 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.15 Safari/537.36" "-"
127.0.0.1 - - [14/Dec/2017:12:01:21 +0800] "GET /test/?k1=v1&k2=v2 HTTP/1.0" 404 571 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.15 Safari/537.36" "-"
127.0.0.1 - - [14/Dec/2017:12:01:31 +0800] "GET /test/?a=a&b=b&k1=v1&k2=v2 HTTP/1.0" 404 571 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.15 Safari/537.36" "-"
127.0.0.1 - - [14/Dec/2017:12:01:35 +0800] "GET /test?a=a&b=b&k1=v1&k2=v2 HTTP/1.0" 404 571 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.15 Safari/537.36" "-"

If you have no args then token is missing the ?

/testk1=v1

This only works if you already have args

If you are missing the ? with even when using $is_args in the URI then making the default token ? should fix this:
set $token "?"
if there are no args then it will be ? else it will be &

If you have no args then token is missing the ?

/testk1=v1

This only works if you already have args

/testk1=v1 it is wrong syntax

If you are missing the ? with even when using $is_args in the URI then making the default token ? should fix this: set $token "?" if there are no args then it will be ? else it will be &

https://gist.github.com/anjia0532/da4a17f848468de5a374c860b17607e7#file-nginx-conf-L38

If has args, set $token "&"; and $is_args=? ,${args}${token}k1=v1&k2=v2 eq ${params}&k1=v1&k2=v2
If not has args, set $token ""; and $is_args='' ,${args}${token}k1=v1&k2=v2 eq ?k1=v1&k2=v2