In this doc, we'll show you How to Create Custom Fields in MailChimp. There are a few additional steps to complete as well -
1. Navigate to Audience, make sure you have the correct Audience selected, then click the Manage Audience drop-down menu and choose 'Signup forms':
2. Now go ahead and select 'Form Builder':
3. Navigate to the Build it tab and under 'Add a Field', click 'Text' to add a new text field:
4. Make the field label 'deadlinetext', select 'Hidden' for field visibility, and add 'deadlinetext' to the Default merge tag value, then click 'Save field':
If you have any questions, please let us know at firstname.lastname@example.org.