For 2018 Fed tax refund, I elected applied it to 2019 estimated tax. Internet said it need to be applied to 2019 1st quarter by 4/15.?

The correct term is first payment, second payment, etc. Not quarter, because two of the payments are in the same quarter and one quarter does not have any.

You can subtract from whatever payment you want and no special form is needed.

The IRS internally, in its system, counts it as having been applied to the first payment, the one due April, which means that less was due then and your payment through EFTPS was more than what was due, so they automatically apply the overpayment to the next regular payment, which is the one in June, so less is due then. This continues through the year until you do the subtraction and pay only what is due. Or if you forget about it completely and make all 4 payments in their normal amounts, then the overpayment is added to your 2019 refund.

Unless you normally make quarterly estimated tax payments, it is treated as if it was payroll withholding evenly throughout the year. The ONLY form is the tax return with which you requested it to be applied to your 2019 taxes.

It will be posted to 2019 as a credit on 04-15-19. How you treat it after that is your business.

Apply it to the second quarter and leave it at that. Refigure your estimates as if that payment had been applied to the first quarter and make the rest of your estimated payments accordingly.