Here is a thread on this matter in particular, which may help. As you say, the Metadata checkbox itself is not terribly useful for this.
Speaking strictly of formatting, you do need the three dashes on a line above and below, like Scrivener would normally put into the .md file. I think empty lines within this block are okay, but I’d keep each row adjacent to be safe, like so:
---
Title: value
Duration: value
Description: value
---