You just need to follow your dashboard setup to apply for Ad Exchange.
Make sure you have filled out the form. If you have not received the email link you need to make sure you have filled out the details correctly (you should be using the email address associated with your Adsense account).
Be sure to check all folders including spam/ junk mail in case the email has been misdirected.
Bear in mind it can take a little while for Ad exchange applications to be processed as Google receives a high volume of these.
Here's some more information that may interest you: https://support.ezoic.com/kb/article/applying-to-google-ad-exchange