You'll want to make the first 2019 estimated tax payment now, then when you file your return, if you are receiving a refund, you may choose to apply it to your 2019 estimated payments if you wish to.
You can make a 2019 estimated tax payment online with IRS Direct Pay. The following FAQ provides additional options for making your estimated tax payments. https://ttlc.intuit.com/replies/3301258
