There is no place to report not offering coverage on the 1120.
ALEs have the Form W-2 reporting requirement and are supposed to supply employees with 1095Cs that are also filed with the IRS using the transmittal Form 1094C. Form 1095C has codes to indicate the type of coverage offered and also a code to indicate no coverage was offered. Code for that is "1H".
Here are 2 pages from the IRS site that might give you a start: Information Reporting for ALEs and Q&A on Employer Provisions. It appears that the IRS hasn't yet finalized a method of assessing the penalty. From the 2nd page linked above, see #27 & 28 where it says that IRS will contact the ALE about the assessment.