[Plugin: SimpleModal Contact Form (SMCF)] captcha test before submitting form

Hi, i want you to help me please to insert captcha test on my form before i submit the form, i know that there is no utility to insert it but my boss wants me to do it and i am blocked at this stage.
Can anybody help me ?? thank you

http://ww.wp.xz.cn/extend/plugins/simplemodal-contact-form-smcf/