How to get variable’s value in split_clients NGINX

I’m using (split_clients) and it works perfectly in most cases.

Ref: https://nginx.org/en/docs/http/ngx_http_split_clients_module.html

split_clients returns a literally string for further use.

But now, I need returned string contains a variable, like this:

http context:

split_clients "${remote_addr}${http_user_agent}${date_gmt}" $my_variable {
    20%     https://example.com/fixed_string/another_fixed_string;
    *       https://example.com/$1/another_fixed_string;
}

server context:

location ~ ^/abc/(.*) {
   rewrite ^/abc/(.*) $my_variable redirect;
}

When I visit https://example.org/abc/something, it redirect to https://example.com/$1/another_fixed_string with $1 is a literally string in URL.

My expectation is $1 acts as a variable, and its value is something, then redirect to https://example.com/something/another_fixed_string

How to implement it?