if you do not have last name on passport, your complete name goes to family name field on the form. since interest form does not let you leave these fields blank, you can rectify it when you fill the application forms (if selected).
check the link -
http://www.cic.gc.ca/english/helpcentre/questions-answers-by-topic.asp?st=4.2
What do you mean by surname?
Your surname is your family name. It is also called your “last name.” When filling out applications, type your surname as it appears on your passport, travel or identity document.
Do not use initials.
If you do not have a family name on your passport, travel or identity document, enter all your given name(s) in the surname field and leave the given name field blank. Do not enter “Not applicable,” “*” or “NA.”