Add text to your password form, modify the HTML form, change the default `Protected:` title prefix. #wordpress

customize-password-protected-text.php

<?php	

/**
* If you want to add text to your password form
*
* @link https://developer.wordpress.org/reference/hooks/the_password_form/
*/
add_action( 'the_password_form', 'll_the_password_form' );
function ll_the_password_form( $output )
{
    $before = ' Before ';  // Modify this to your needs!
    $after  = ' After ';   // Modify this to your needs!
    return $before . $output . $after;
}

/**
 * Modify the HTML form directly
 * (The password field is limited to to 20 characters)
 *
 * @link https://developer.wordpress.org/reference/functions/get_the_password_form/
 */
add_filter( 'the_password_form', 'll_override_the_password_form' );
function ll_override_the_password_form( $form = '' ) {
    global $post;
    $label = 'pwbox-'.( empty( $post->ID ) ? rand() : $post->ID );
    $form = '<form action="' . esc_url( site_url( 'wp-login.php?action=postpass', 'login_post' ) ) . '" method="post">
    ' . __( "To view this protected post, enter the password below:" ) . '
    <label for="' . $label . '">' . __( "Password:" ) . ' </label><input name="post_password" id="' . $label . '" type="password" size="20" maxlength="20" /><input type="submit" name="Submit" value="' . esc_attr__( "Submit" ) . '" />
    </form>
    ';
    return $form;
}

/**
 * Change the default Protected: title prefix
 *
 * @link https://developer.wordpress.org/reference/hooks/protected_title_format/
 */
add_filter( 'protected_title_format', 'll_protected_title_format' );
function ll_protected_title_format( $format )
{
    $format = __( ' Members only! %s ' ); // Modify this to your needs!
    return $format;
}