Using existing extra SIM to sign up for $20 90 day promo plan

Since we get a couple extra SIMS, how would your friend be able to use that SIM to get on the new promo? Or would they have to order new SIM - resulting in having to wait to sign up plus wasting resources having to send another kit out.

Oh, I took out both SIM 1 and SIM 2 and it looks like the activation code is printed on the back, so I guess just use that then choose the $20 for 3 mo promo plan to sign up, nice!

I saw this in the FAQ for the promo:

"Can I get this deal if I purchase a Starter Kit?

Yes! After you’ve taken us out for a test drive, you can purchase our promo deal in our free app. But this is a limited time offer so get it before it ends!"

That seems to imply your friend would need to do it via the app used to start the free trial. I did try going through the steps to purchase the $20 plan/SIM directly on the website, but don't see a way to designate "I already have a SIM".
Trying to skip ordering a physical SIM card by going straight to 'Activate' shows a message that if you're activating a Starter Kit SIM to do it through the app.