How can I add reCAPTCHA to my form
You need to have field with selected “reCAPTCHA” type in the plugin settings (“Contact Form” > “Settings” > “Form” tab > “Fields Settings” section > “Captcha” field type > “Captcha Type” drop-down in the field advanced options) and fill in API-key pair here: “Contact Form” > “Settings” > “reCAPTCHA” tab > “General Settings” section.
More information you can find here.