There are two standard methods for receiving consent from your recipients. The first and only recommended way to get consent is a sign up form (email your recipients with a link to go fill out that form on your website).
An alternative is to use a permission pass email. It's imperative that the content you deliver in this permission pass be very similar to normal content delivered regularly. A simple message asking recipients to confirm their interest will likely be perceived (by the recipients' domains) as delivering to a list that has not been given permission. As such, the delivery of the said mailing may suffer. Providing your normal content and permission pass link will help alleviate this problem.
The idea of a permission pass email is to send out a mailing with a link that reads "yes, I consent" (or however you feel you need to word the agreement). That should link that to a page on your site. The URL is irrelevant, but it should take recipients to a page acknowledging that they have now provided consent to be mailed. The important part is to provide positive, clear feedback explaining that the recipients' necessary actions are now complete.
Using the ReachMail link tracking system, you can then collect the information from recipients who have clicked on that consent link, creating or appending them to a different sending list. The tracked links section of a ReachMail a campaign report provides a convenient link for accomplishing this quickly.
From the Link Tracking & Heatmap tab of your reports you will find the consent link select from the Action column the ellipses for More options. You will then select Copy Clickers to list. From here you can create a new list or append the contacts to one of your current lists.
It is recommended that you wait 24-48 hours before collecting these "clickers" to give latecomers a chance to view your message and provide consent.