How can I extract the URL of a link from a post?

Exist some WordPress function that retrieve the list of links (in my
specific case only one) that are into a post?

Check out the core wp_extract_urls() function:

$urls = wp_extract_urls( $content );

that will extract urls from any given content.

Example

$content="Two test sites: <a href="http://foo.tld">Foo.tld</a> and https://bar.tld";
$urls    = wp_extract_urls( $content );
print_r( $urls  );

with output:

Array
(
    [0] => http://foo.tld
    [1] => https://bar.tld
)