Looks like I found some info in a youtube video. I'm not sure about whether you can post links here, so instead I will post the title of the video: "
How to open the dipstick oil cap of a Weedeater Lawnmower
"
He shows how to use an 11/16" wrench, or any wrench that fits, turn counterclockwise...
The mower doesn't have a dipstick assembly and the customer service said to take it back. Still, I'm curious about what it is, it shows in the Owners manual. Edit: it seems like another kind of dipstick, but I'm unfamiliar with how to access it.