How to add the value of a User_ID to an FK?

Verify that the user is logged in, otherwise he will receive a script "You must be logged in to comment".

If the user is logged in, then he should get the id of that user and move to autor_coment (which is a FK of the comments table)

That's it. As I'm starting and I did not find any examples of codes like this, I decided to ask for help here. However, when registering the comment I can not leave autor_coment null, since it is a FK . So I wanted to get the id of the logged in user and move to autor_coment .

In short ... when I post a comment I can not leave this to FK autor_coment null. So I want to pass the value of the user id logged in to this FK .

Until then, I have this code to register the comments:

<?php
if(isset($_POST['comentar'])){
            $autor_coment       = trim(strip_tags($_POST['autor_coment']));
            $comentario             = trim(strip_tags($_POST['comentario']));

    if(empty($autor_coment)){
        echo "<script>alert('Preencha todos os campos para comentar.'); history.back();</script>";
    }elseif(empty($comentario)){
        echo "<script>alert('Preencha todos os campos para comentar.'); history.back();</script>";
    }else{

    $insert = "INSERT into comentarios (autor_coment, comentario) VALUES (:autor_coment, :comentario)";

        try{
            $result = $conexao->prepare($insert);
            $result->bindParam(':autor_coment', $autor_coment, PDO::PARAM_STR);
            $result->bindParam(':comentario', $comentario, PDO::PARAM_STR);
            $result->execute();
            $contar = $result->rowCount();
            if($contar>0){
                echo "<script>alert('Seu comentário foi enviado com sucesso!')</script>";
                header("Location: ../index.php");
            }else{
                echo "<script>alert('Erro! Não foi possível comentar')</script>";
            }           
        }catch(PDOException $e){
            echo $e;
        }   
    }
}
?>

<br>
    <form action="" method="post" enctype="multipart/form-data">

                    <p>

                        <input 
                        style="width: 400px;" 
                        type="" 
                        name="id"

                        required
                        placeholder="ID">

                        <br><br>

                        <input 
                        style="width: 400px; height: 150px;"
                        type="text" 
                        id="comentario" 
                        name="comentario" 
                        value="" 
                        required
                        placeholder="Digite seu comentário"
                        /> 

<br>
                    <a style="padding-left: 280px;">
                        <input 
                        style="width:120px; height:40px; background:#333; color:white; border:none;"
                        type="submit" 
                        name="comentar" 
                        class="w3-button w3-black w3-section"
                        value="Comentar"
                        />
                    </p></a>

    </form> 

I'm getting the user ID in the comment and I'm not verifying that the user is logged in or not. So there are two problems that I have no idea how to solve.