to add server side validation go to WebAssist -> Forms -> Validation -> Server Validation.
Select the validation type, then click the lightning bolt next to server variable and select the form element to validate, then click the plus button to add it to the validation list. repeat for each element you wish to validate.
to show a validation message, type the message on your page and select it. Go to WebAssist -> Forms -> Validation -> validation Show if. select the validation that should trigger the error to show and add it to the list.
add a message for each validation you add to the server validation behavior.
on the data bridge support page, see the "Add server validation to your web forms" tutorial for details.