If you’re okay with paying for a plugin, here’s one that does exactly what you need:
https://querysol.com/product/contact-form-7-redirection/
Otherwise you need to write some code yourself, the documentation is here:
https://contactform7.com/redirecting-to-another-url-after-submissions/
The code itself needs to be extended a bit, e.g.
<script>
document.addEventListener( 'wpcf7mailsent', function( event ) {
if (jQuery('#yourdropdown').val() == '0') {
location = 'http://example1.com/';
} if (jQuery('#yourdropdown').val() == 'R9999') {
location = 'http://example2.com/';
}
}, false );
</script>