If you leave the field blank, it should default to withdrawing the funds on today's date, since it's the tax deadline.
Since you are filing on the busy April 15, 2019 deadline, the IRS may not be able to debit your payment until after the deadline, due to high e-filing volume. In addition, it can take your bank another few days to post the debit to your account.
No worries; as long as you filed your return or extension on or before April 15, 2019 and you selected April 15, 2019 (or earlier) as your payment date, you won't get penalized for late payment.
In the meantime, keep an eye on your bank account — if you still don't see the debit 7–10 days after your return has been accepted, call IRS e-file Payment Services at 1-888-353-4537 or contact your state tax agency, as appropriate.