Custom Rewrite Rule Removing Query String

I’m trying (and failing) to use rewrite rules to redirect shorter/prettier URLs to the right page AND include values in the query string.

WP is installed at https://example.com, and the plugin creates a page called ‘foo’ which uses a value ‘bar’ passed in the query string to generate the content from custom database tables.

So an example full URL would be:

https://example.com/foo/?bar=XXXXXX

The ‘bar’ value would always be a string of between 6 and 8 alphanumeric chars with at least 1 dash(-).

I want to use rewrite rules (but am open to other suggestions) so that a shorter URL can be used, such as:

https://example.com/f/XXXXXX

I have tried using add_rewrite_rule as follows:

And, when I flush the rewrite rules by clicking save on the Permalinks Settings page in the Dashboard, the rule is added to the htaccess file just fine:

However, when I try to access:

https://example.com/f/XXXXXX

the page is just redirected to:

https://example.com/foo/

and is missing the needed bar=XXXXXX querystring so the relevant content does not load.

Can anyone please point out where I am going wrong wiht this or suggest an alternative?