Hello JoeP893 ,
Copy the email's permanent link/share URL, and include that with your SMS message. Remember, this link is only generated when an email has been at least scheduled, and isn't live/accessible until the email has actually gone out. Therefore, your steps would be:
Create and schedule the email
Copy the email URL from the details page
Create your SMS campaign and add the email's URL
Schedule the SMS to either send at the same time or just after the email goes out. I recommend after, in case there's an issue that occurs with the email that prevents it from sending. That way, you can still have time to unschedule the SMS to prevent a non-functional link from being sent out.