从一个表中获取USER ID并将其添加到另一个表中

I hope you can help me. I have two tables. One table has the users who register on my website. The other table contains the messages that are sent via a form.

How can I transfer the user ID to the message so that I know which registered user wrote the message? Is that possible somehow, I don't know where to start?

My current status is that I only output the messages, but without USER ID or Username. I'd like to print this out as well.

<?php


        if(isset($_POST['submit'])):
            $betreff = $_POST['betreff'];
            $nachricht = $_POST['nachricht'];

            $absenden = $db->prepare("INSERT INTO kontakt (betreff,nachricht,datum) VALUES (?,?,NOW())");
            $absenden->bind_param('ss',$betreff,$nachricht);
            $absenden->execute();
        endif;

$abfrage = $db->query("SELECT * FROM kontakt ORDER BY datum DESC");

echo 'Es wurden '.$abfrage->num_rows.' Nachrichten gefunden!<br>';

while($ausgabe = $abfrage->fetch_object()){
echo '
<b>Betreff:</b> '.$ausgabe->betreff.' <br>
<b>Nachricht:</b><br> '.$ausgabe->nachricht.' <br>
<hr>
';
}
include 'footer.html';   
        ?>

It would have to work somehow via inner join, wouldn't it? Who can help?

Output Messages:

if(isset($_POST['submit'])):
                $betreff = $_POST['betreff'];
                $nachricht = $_POST['nachricht'];

                $absenden = $db->prepare("INSERT INTO kontakt (betreff,nachricht,datum) VALUES (?,?,NOW())");
                $absenden->bind_param('ss',$betreff,$nachricht);
                $absenden->execute();
            endif;

$abfrage = $db->query("SELECT * FROM kontakt ORDER BY datum DESC");

echo 'Es wurden '.$abfrage->num_rows.' Nachrichten gefunden!<br>';

while($ausgabe = $abfrage->fetch_object()){
    echo '
    <b>Betreff:</b> '.$ausgabe->betreff.' <br>
    <b>Nachricht:</b><br> '.$ausgabe->nachricht.' <br>
    <hr>
    ';
}

My structure:

Table users:

ID (Primary Key) 
Username (Index) password created_at updated_at 

Table contact:

id (primary key) topic message date