Add target="_blank" to external links.

dynamic-html-attributes-acf-link.php

<?php
/**
* https://www.php.net/manual/en/function.substr-replace.php
* https://www.advancedcustomfields.com/resources/acf-load_value/
*/

/* Add target _blank to external links with ACF field */
function add_blank_target_to_external_links($value, $post_id, $field) {
    // Check if the link is external
    if (preg_match('/https?:\/\/(?!' . preg_quote($_SERVER['HTTP_HOST']) . ')/i', $value)) {
        //3 is index of h in href; 0 since we're not replacing text.
        $value = substr_replace($value, 'target="_blank"', 3, 0);
    }

    return $value;
}
add_filter('acf/load_value/key=field_601adc4d344be', 'add_blank_target_to_external_links', 10, 3);