TT says 2/18/2021.  the actual 1st quarter estimate is not due until 4/15/2021.   you have the option of using IRS Direct Pay and avoid having to mail in the voucher and check. you have some leeway in selecting the payment date and at the end you can print a confirmation copy for the payment.  you can even cancel the payment if you do it by the date given to you. 

https://www.irs.gov/payments 

Thank you.  That's all I needed to know.

I want to send a check and the 1040Es form.  Can you send me new 2021 Form 1040-es forms?

The 2021 forms are not out yet, they will not be out until next tax season. 

If you want to file by mail and send in the check, you can mail everything to the address printed on the instructions. 

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