How to add Button to firstrunwizard that closes the wizard


I am totally new to ownCloud, php and javascript.
My goal is to adapt the firstrunwizard to display a button. Only the click of the button closes the firstrunwizard.

So fare I was able to add a button. The functionality worked only after I disabled the csp (content security policy) of my browser.
Then I tried to write the code by the rules of the csp shown here:
But apparently I did something wrong.

This is what I have so fare:

    <div id="firstrunwizard">

    <h1>Some Text.</h1>
    <p>Some Text.</p>

            <img src="/core/img/somelogo.svg">
    <script> function myFunction()
            alert("Hello World!");
    window.onload = function () 
            myAnchor = document.getElementById("accept_license_btn");
            myAnchor.onclick = function(e)
            myFunction(); // e.preventDefault();
    <p> <button id="accept_license_btn">Some Text.</button> </p>

    <p class="footnote">
    Some Text.


I know that I am still working inline but when using the browser debugger the csp does not seem to care about that. Also I know that it only displayes a 'Hello World!'. At the moment I am doing a lot of try and error.

I would appreciate some help.