Add segment to URL in htaccess

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Go to Settings/Permalinks and add the static string to to the permalink structure:

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No need to touch the .htaccess.

Update

To make WordPress sending a hash # for its post permalinks you have to filter pre_post_link to make the structure '#%postname%' and post_link to remove trailing slashes:

add_filter( 'pre_post_link', function( $permalink ){
    return '#%postname%';
});

add_filter( 'post_link', function( $permalink ){
    return rtrim( $permalink, "https://wordpress.stackexchange.com/" );
});

Be aware, you will ruin your site with those permalinks.

See also Change the link URL in default RSS feeds