Help with Nginx proxy rewrite rule

[lenovo]

I use OPNsense Nginx as a proxy for a webserver.
The webserver has a site in a folder (myfolder) in the html root: /var/www/html/myfolder
This all works as expected. When I use a browser it shows https://mywebsite.com/myfolder in the address bar.
Now I'm trying to only rewrite the url with Nginx so the browser will show https://mywebsite.com but NOT redirect.
This is the OPNsense-Nginx config I tried:

Original URL Pattern (Regex): ^.*$
New URL Pattern:  https://mywebsite.com
Flag: (tried all options)

In location this rule is selected.
The result works (address bar only shows https://mywebsite.com), but browser errors with too many redirects.

Newbie here with Nginx and Regex, so I expect I'm doing something stupid.
Can some kind soul enlighten me and show me the right Regex syntax / Nginx url rewrite config?
   

[Fright]

imho url rewrite is not needed in this case
you can try to add  "/myfolder/" Path Prefix in "/" location settings

[lenovo]

Thanks for your reply and help!
This did not work. /myfolder gave a page not found and /myfolder/ gave an index listing.
However, what I failed to mention is that I am searching for a solution for the url to always stay the same in the browser (https://mywebsite.com), whatever folder is used bij the target webserver.
So A solution which works with more then one specific folder 

[Fright]

then i don't understand your end goal and/or backend settings, sorry..