Add the code below to the Custom CSS field on your Form widget’s Appearance tab:
[class*='FieldsGrid__Row-sc']:nth-child(2),
[class*='FieldsGrid__Row-sc']:nth-child(3),
[class*='FieldsGrid__Row-sc']:nth-child(10) {
border-bottom: 1px solid rgba(17, 17, 17, .3);
margin-bottom: 10px;
}
Replace the numbers in the brackets with the number of the row after which you’d like to add a divider