Rewrite cname host mapping url paths with nginx

There is currently an app that is using cname host mapping with a third party app

the hostname for the third party app is mycompany.partner.com and the current cname host map under my domain help.mycompany.com

so current routing is below

users => cloudflare DNS(help.mycompany.com) => cname host mapping(mycompany.partner.com) => partner app

now i want to do this

                                                             => cname host mapping(mycompany.partner.com) => partner app
                                                            |
users => cloudflare DNS(help.mycompany.com) => my nginx =>  |
                                                            |
                                                             => my frontend app

is this possible?

So basically i want all traffic to come to my own app via nginx now and then i route some traffic based on url path to the third party app and others to my frontend app

how can i achieve this with nginx? below are the url paths i want to route

this routes to my frontend app

help.mycompany.com/app/test1                  => http://localhost:500/app/test1  
help.mycompany.com/app/test2/test3            => http://localhost:500/app/test2/test3
help.mycompany.com/app/parameter?key=check    => http://localhost:500/app/parameter?key=check

this rewrites/routes to partner app

help.mycompany.com/app/partner1               => https://mycompany.partner.com/app/partner1 
help.mycompany.com/app/discuss/check          => https://mycompany.partner.com/app/discuss/check

and all other paths 

location block to route all url paths to the frontend app is below

location ^~ / {

    rewrite ^/(.*)$ /$1 break;
    proxy_pass http://localhost:500;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $host;
    proxy_redirect off;
}

but now i need to split some url paths to the partner app as stated above

so basically all traffic will pass through nginx as i will point DNS for help.mycompany.com to now point to my nginx reverse proxy and then routes and rewrites the url

Thanks