functions.php

<?php

add_filter( 'get_post_metadata', 'my_custom_url_prepend_value', 99999, 4 );

/**
 * Get Meta Filter
 *
 * Filter the get_metadata function to return specific value we 
 * customize.
 *
 * Null should be returned to allow get_metadata to proceed and
 * actually get the meta.  If anything other than null is returned
 * that is what will be returned by get_metadata and used for output.
 *
 *
 * @see https://developer.wordpress.org/reference/hooks/get_meta_type_metadata/
 *
 * @param null|array|string $null      The value get_metadata() should
 *                                     return - a single metadata value,
 *                                     or an array of values.
 * @param int               $object_id Object ID.
 * @param string            $meta_key  Meta key.
 * @param string|array      $single    Meta value, or an array of values.
 *
 * @return mixed
 */
function my_custom_url_prepend_value( $null, $object_id, $meta_key, $single ){

    /**
    *   You MUST prepend metakey with underscore!  In example below, our meta keys 
    *   are company_website, facebook_url, application_url
    */
    $my_url_fields = array(
        '_company_website',
        '_facebook_url',
        '_application_url'
    );

    // URL to prepend
    $my_url_prepend = 'https://mysite.com/out.php?url=';

    if( in_array( $meta_key, $my_url_fields ) ){

        // We have to remove filter to prevent loop back to our function
        remove_filter( 'get_post_metadata', 'my_custom_url_prepend_value', 99999 );
        // Pull value from database/cache
        $actual_value = get_post_meta( $object_id, $meta_key, true );
        // Now we add it back for any additional fields that are called
        add_filter( 'get_post_metadata', 'my_custom_url_prepend_value', 99999, 4 );
        
        // Make sure there is a value, it's a string, and a valid URL
        if( ! empty( $actual_value ) && is_string( $actual_value ) && filter_var( $actual_value, FILTER_VALIDATE_URL) ){
            // Return the prepend value, plus the actual value
            return $my_url_prepend . $actual_value;
        }
        
    }

    // Return null to let WordPress pull value from database
    return $null;
}