Redirect traffic to plugin

You can use the init hook to check the $_REQUEST or $_GET super-global variable for your token value and then redirect to pages that are present via WP Core:

• home_url
• wp_login_url

If you pass additional parameters to these URLs, such as token_valid=true you can use that along with conditional tags like is_front to conditionally display thank you message.

function wpse290114_init_validate() {
    if(isset($_REQUEST['token']) && !empty($_REQUEST['token'])){
        // validate the token and set $token_valid to 'true' or 'false'
        if($token_valid){
            wp_safe_redirect(add_query_arg( 'token_valid', $token_valid, home_url() ));
        } else{
            wp_safe_redirect(add_query_arg('token_valid', $token_valid, wp_login_url() );
        }

    }
}
add_action("init", "wpse290114_init_validate");

The position and display of the thank you message depends on the theme.

function wpse290114_thankyou(){
    if(isset($_REQUEST['token_valid']) && 'true' === $_REQUEST['token_valid'])){
      echo '<p class="message--thankyou">Thank you for validating.</p>';
    }
}
add_action("your_theme_hook", "wpse290114_thankyou");