You will send your payment marked as a "deferral payment" to the IRS by December 31, 2019. The IRS has not yet created a code for this in the electronic payment system "EFTPS" or created a voucher for this purpose.
We expect this to be released in the coming months.
Keep an eye on this link for updates: What employers need to know about deferral of payroll taxes. Note that while this link refers to "employers", it also applies to those who are self-employed.