Form ID of the form is "webform_client_form_21". CAPTCHA module validates Form IDs using /^[a-z_]+$/. then Goto sites\all\modules\captcha and Open captcha.admin.ini file add modify line no 255,400 line preg_match('/^[a-z0-9_]+$/', to preg_match('/^[a-z0-9_-]+$/', and save file. Created a form with the webform module, checked "Add CAPTCHA administration links to forms", visited a form I created with the Webform module, clicked the link "Place a CAPTCHA here for untrusted users." clicked "Save". now you are done but to work captcha just modify form id for exaple if you got form id from webform webform-client-form-1 then enter form id to captcha webform_client_form_1 :)