The asterisk can be added as a label in the design. The validation has be done in the code using conditional statements (if - else). You will need to compare the textbox value. If it is blank (i.e. value is "") you can show a error message.
https://support.plezmo.com/support/solutions/articles/65000068421-3-2-condition
Sultanmalik001
how to required a textbox...(how to mark asterisk on textbox..)