How to get the key from a keygen?

Question

I have an HTML document with a form and I want to send a random key to the server. To do this, I’m using the element <keygen>.

Before the data is sent, I need to get the value of this element <keygen> to store the generated key locally. I tried the following code:

function getKey() {
    const keygen = document.getElementsByTagName("keygen")[0];
    sessionStorage.token = keygen.accessKey;
}

document.onsubmit = getKey;

The problem is that it always returns an empty string instead of the generated key. What should I do to get the key before sending the form?