We have a YouTube video that describes this: https://www.youtube.com/watch?v=rqQpiSisY1E
Please be aware that the drop down list is not editable.
As you are building a proposal, click on Add next to Custom Sections
You need a title, then you can add content. You can paste a link to a YouTube video, or items from your google drive, as long as it's a link that anyone can view.
For files in my google drive, I can right-click, choose Get link
On the form that pops up, choose Anyone with the link (it might say restricted and that won't work), then click on Copy link, and then paste the link (Ctrl V) right into your custom section.
You can toggle the switch to save it as a template and it will be on the drop down for you to pick while creating a proposal. Please be aware that the drop down list is not editable. Users cannot save over or remove anything that is on that list, we can only add to it, so be judicious about toggling the switch to save as a template.