Hey! So, I use a little trick to do that. Whenever you do a fieldset, it wants a title. It defaults to Fieldset, but you can make it whatever you want. To make it appear with no title, I use a 'whitespace' character. You can find one here:
Copy and paste blank Unicode characters like the zero-width space and braille blank — and learn about when to use them.
qwerty.dev
So! The exact code you would use would look like this:
Code:
[fieldset=45|BCENTER| ] TEXT GOES HERE [/fieldset]
So, just to explain what you're seeing:
'45' is the width of the fieldset. This can be adjusted up and down until you're happy with it.
BCENTER positions the fieldset; in this case, it centers it.
The last spot, after BCENTER|, is where the whitespace goes. Copy a whitespace symbol from the website above (any of them will do, I use thin ones just because I know the alt code off the top of my head) and paste it there. That should do it!