Making a button in iframe trigger a button on parent


So I have this form that is iframed in, and I need it to trigger a button on the page. I’ve tried using javacript snippets I have found on stack overflow, but I can’t make it work. I know nothing about Javascript, so I’ve probably done this all wrong.

The button in Iframe is named “bttnsubmit”, and the button on the page is named “bestilling”

Inside embeded:

<iframe height="100%" width="100%" padding="0px" src="" style="border:none;"></iframe>

Inside header:

<script src=""></script>