I want to add contact directly to one list from my website using API without listing other lists.
What hidden field should I add to form to achieve it?
We do not allow you to submit any hidden fields through our normal sign up form. If you do not want to show the lists you can do that through the grow my contacts area under customize your form. From there you can click on lists and change what is visible. Otherwise you would need to create a custom join my mailing list page through our API.
Please let me know if this helps.