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
)