Many thanks for reaching out to us!
Supporting custom fields at Subscription level is in our product roadmap. Our engineering team is currently working on the task and we are planning to roll it out by end of Q2, this year.
We'll certainly reach out to you once it is made available.
Hope you're doing well.
We would like to inform you that custom fields are now supported at subscription level. You could create them at subscription level and have them appear only for subscriptions.
You will find the option to create the fields under "Settings > Site Settings > Custom Fields" and you can choose them to appear on hosted pages under "Settings > Hosted Pages Settings > Field Configurations".
For existing subscriptions, you can use the "Change Subscription" option to update the custom field.
Please try and let us know if you have any questions.
Can you please add a custom field to 'Subscriptions' called "Tracking ID" text field