Subscriptions with checkout API, preview is blank

I figured out I need to pass in the last phase of the subscription when creating the payment link…

One last question I promise, I think…

I’d like to be able to pass in an ID for the user when creating the payment link for a subscription. Additionally, I’d like to receive this user ID on my end when subscription events come in through the webhooks. Does anything exist like that currently?

Thanks,
Richard