That same question has been asked before. May be you should search the forum a bit more. The post can be found here: https://forum.mendix.com/questions/15932/User-Email-Activation
For this to achieve, you cane use the e-mail module for sending an e-mail. For the link you can use the deeplink module. Via this module it is possible to open a specific form of attach specific logic to an URL. So in your case, the user can then complete the activation.