If you have a notice from the IRS asking for the missing 2019 1040 you should send it to the address indicated in the notice. See
https://www.irs.gov/businesses/small-businesses-self-employed/filing-past-due-tax-returns
Otherwise I would mail to the address given here based on your current state of residence (not where you lived in 2019):
https://www.irs.gov/filing/where-to-file-addresses-for-taxpayers-and-tax-professionals-filing-form-1...
Send the return certified mail and get a postmark stamp at the post office. You do not need a return receipt. That receipt is your proof of filing.
Be aware that the IRS paper return backlog is amazingly long. It wouldn't surprise me if it took them a year or two to process it.
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