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You can do this Injecting a Custom CSS Code with the next steps :
border-radius : 0px !important;
-webkit-border-radius : 0px !important;
-moz-border-radius : 0px !important;
-o-border-radius : 0px !important;
-ms-border-radius : 0px !important;
Or you can go to Designer and paste the above code as well.
Here is my sample form, you can clone it if you want :
I hope this helps.