Need help trying to change the send button to square edges and no border

Hey very new to all this. I'm trying to change the button on this form. It's a Dubsado form embedded into a Showit page.
I want it rectangle with sharp edges not rounded and no border. Dubsado does some control over the button.
It's on this page -

https://photoform.com.au/start-up-package-contact

Thanks!