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?