Anyone submitting an application, proposal or report to the NIH must include the PubMed Central reference number (PMCID) when citing applicable papers that they author or that arise from their NIH-funded research.
Try PubMed Central ID Converter to see if an article has a PMCID.
What is the difference between a PMID and a PMCID?
For further information about the actual policy see NIH Public Access Policy details.
For submission Methods A and B use PMC-In Process in place of the PMCID because you will not have an NIHMSID to enter.
For submission Methods C and D use the NIHMSID until the PMCID is available. The NIHMSID is assigned during the submission process.
Once you are assigned the PMCID number refer to the above box to add the PMCID.