This is done using the Like Entry validation in the server Validation server behavior.
add the password field, and the confirm password field to your page:
Insert -> Forms -> Password
the add the server validation behavior, WebAssist -> Forms -> Validation -> Server Validation
select the Like Entry validation type, click the lightning bolt next to server variable and select the password form field. then click the plus button, you will prompted for the comparison value, click the lightning bolt and select the confirm password field.