To receive Apple Pay payments from your clients when using Joist Payments with Stripe, you'll need to log into your Stripe account first and add some info. This should only take a minute or two.
Log into your Stripe account by going to https://dashboard.stripe.com/dashboard
Go to Settings > Payments
On the Payments page, select the tab named Payment methods domain
Click the Add new domain button
Enter client.joistapp.com and save
And you're all done! Your clients should now see Apple Pay as a payment option at checkout.