All used URLs are examples.
My goals is to:

1. Redirect mydomain.com/demo?psswd=demo to a external domain like demo.platform.com/80201bf0-c857?psswd=demo
2. The redirect would only redirect the mydomain.com/demo folder, not the main website.
3. Without changing the original URL displayed to the user.

I can get step 1 and 2 to work, but not step 3.

What does work:
Placed an .htaccess file in the mydomain.com/demo folder.

RewriteEngine On
Options +FollowSymLinks
RewriteCond %{REQUEST_URI} ^/demo
RewriteRule ^(.*)$ https://demo.platform.com/80201bf0-c857 [L]

I tried to change the [L] to [P] or [P,L] so that the URL wouldn't change, but then the redirect doesn't work anymore and I get a error (page not found)

RewriteEngine On
Options +FollowSymLinks
RewriteCond %{REQUEST_URI} ^/demo
RewriteRule ^(.*)$ https://demo.platform.com/80201bf0-c857 [P,L]

a. Is that what I want possible?
b. What do I need to change in the .htaccess file?