Information sharing through online WOM has become increasingly important for businesses. Despite the popularity of online referral programs, little is known about how firms can optimally design call-to-action to encourage referrals, as well as the motives underlying those referrals. In collaboration with a large US based online platform specialized in photo processing, we conduct a large randomized field experiment involving 100,000 customers to identify the causal effect of three types of call-to-action for referral (egoistic, equitable and altruistic) that are widely used in practice. Our experiment shows that, surprisingly, 'altruistic' call-to-action leads to highest likelihood of referral and best referral outcomes. Such altruistic framing is more effective for customers who had repeated purchases in the past and who reported higher Net Promoter Score. Also, we find that the effect of altruistic framing decays fast after customer's purchase. In this way, our study provides direct managerial implications to firms on the optimal design of call-to-action for referral campaigns (how, to whom and when to send call-to-action for referral). We also show that altruism is an important driver of online referral among customers and how such motive leads to referral decision and referral outcomes. Finally, we discussed the key differences and complementarity between call for referral and call for purchase, and offer guidance on firm's integrated marketing communication strategy.