Prior to submitting the application, contact globalsupport@icann.org.
Once the application has been submitted, an applicant can use the "Create Inquiry" functionality to submit an application-specific question. For questions related to the application, go to the Application summary view and use the "Create Inquiry" button. To ask a question related to the organization details, or financial capability, go to the Organization summary view and use the "Create Inquiry" button there. Note that general inquiries (not related directly to the application) should go to globalsupport@icann.org.
Responses can be found by clicking "Messaging Inbox" at top-left, next to the Home button. Within the messaging inbox, click "All Inquiries" and/or clear filters to find the inquiry you are looking for.