NIH Manuscript Submission System
The NIH Manuscript Submission system (NIHMS) is a platform developed by the National Institutes of Health (NIH) to manage the submission of peer-reviewed manuscripts accepted for publication in a journal to PubMed Central (PMC), in accordance with the NIH Public Access Policy. The system allows authors, principal investigators, and publishers to submit manuscripts for conversion into a format suitable for PMC, which is a free repository of biomedical and life sciences literature.
NIHMS (https://www.nihms.nih.gov/login/?next=/submission/) allows you to submit an electronic version of your author accepted manuscript for inclusion in PubMed Central.
Below is a screen capture to show the NIHMS submission process

The NIHMS System User's Guide to Submitting a Manuscript - This user’s guide gives you step-by-step instructions on how to submit and approve manuscripts so that they may be placed in PMC for permanent archiving.
NIHMS System Slide Show Help - The NIHMS System Slide Shows demonstrate how to perform various actions in a step-by-step fashion and are illustrated with screen captures.