Hi @Pbalazs89,
The code should be exactly as this since it’s added in text element,
<script type='text/javascript' src='//www.opentable.com/widget/reservation/loader?rid=164868&type=standard&theme=wide&iframe=true&overlay=false&domain=com&lang=en-US'></script>
And based on your video, there is extra </div>
, please remove it.
What’s Ruenel provided is correct too if you added it in header’s custom JS instead of text element. And it’s moving to the right due to left 50% margin added to the text element. If you plan of adding narrow width to the text element then it should be applied as max-width instead of margin.
I went ahead and fix this, it should serve as a sample too 
Before

After

I also changed the flex setting of the container to make sure it’s centered. I changed it from Standard to Fill Space.
Hope this helps.