Running into a "Cannot assign... must be a ...instance" error

New to Django.

I'm trying to set up a simple comment form where I can add a title for the comment, content of the comment, and attach the username of the user that is currently signed in and posting it.

I run into the following error when running it:

Cannot assign "'john'": "Entry.author" must be a "User" instance.

 I think I understand what's happening here, however I'm having a hard time figuring out how to implement it properly.

Here's my model code:

class Entry(models.Model):
    title = models.CharField(max_length=60)
    content = models.TextField(max_length=500)
    author = models.ForeignKey(User, on_delete=models.CASCADE, null=True)
    
    def __str__(self):
        return f"{self.id}"

Here's my view code:

@login_required
def add(request):
    if request.method == "POST":
        entry = Entry(
            title = request.POST["title"],
            content = request.POST["content"],
            author = request.POST["username"]
        )

        entry.save()

        return render(request, "articles/index.html", {
        "entries": Entry.objects.all()
        })

    else:
        return render(request, "articles/add.html", {
            "username": request.user
        })

And here's my form code in my template:

{% block body %}
    <h1>New Entry</h1>
    <form action="{% url 'add' %}" method="post">
        {% csrf_token %}
        <input type="hidden" name="username">

        <label for="title">Title:</label>
        <input type="text" name="title" placeholder="Entry Title">
        <br>
        
        <label for="content">Content:</label>
        <textarea name="content">
        Content goes here
        </textarea>
        <br>

        <input type="submit" value="Publish">
    </form>
{% endblock %}

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