You can add a cornerRadius query parameter, which can take four possible values: s, m, l, or a number in pixels.
A few examples:
https://badgers.space/badge/look/I'm%20slightly%20rounded?co...
https://badgers.space/badge/look/I'm%20rounded?cornerRadius=...
https://badgers.space/badge/look/I'm%20even%20more%20rounded...
In general, I'd recommend going with s or m. Anything larger than l will look weird, and anything less than s will be barely noticeable.
The actual values are: s: 2px, m: 4px, l: 6px.