How can I use an iframe element as my header media?

I am currently using the twenty-seventeen theme, which allows me to display a youtube video on the homepage and images on other pages.

Instead of a youtube video, I wish to display an iframe on the homepage in the header media. In WordPress’ customise header menu I only have the option of entering a YouTube video url, or selecting an image

I have used the inspect element feature on my browser to replace the youtube video with the iframe in my browser and it looks perfect – how can I achieve this in the WordPress editor?

On Google, I’ve seen some vague posts about editing the theme etc, but nothing specific to what I’m trying to achieve. I’m happy to edit the PHP files but I’d need some pointers in the right direction, as I can’t see anything useful in header.php.