Use the Server Validation from Data Bridge.
Go to WebAssist -> Forms -> Server Validation to add the server validation behavior.
from the validation type, select Required.
For the server Variable, click the lightning bolt icon and select the form element. Then click the plus button to add it to the list
repeat this for each form element to validate, then click Finish.
After adding the server validation server behavior, you will want to add error messages to the page to show when validation fails.
Type the error message, then go to WebAssist -> Forms -> Validation -> Validation Show if. Select the validation that should trigger the error and click the plus button, the click OK to apply the behavior.
repeat for each error message.
to retain the form values when validation fails, select a form element in design view. in the property inspector, click the lightning bolt next to initial value and select the corresponding binding from the Validated Entries Group.