There's no way to add an official reward to your survey in SurveyMonkey, but there are some ways you can add incentives to your survey.
You can host a coupon or special offer on your own site and change the Survey End Page option to redirect to that URL when a respondent completes your survey.
If you want to offer a sweepstakes or raffle incentive in exchange for taking your survey, you have to set this up outside of SurveyMonkey. For example, after you collect all your responses, you'd need to select a winner manually once you collect all your responses.
For best practices, see our blog: How to Offer Survey Incentives Without Sacrificing Good Data (English only)