there is no penalty if you meet one of these tests
1) if your withholding (WH) (and any TIMELY estimated tax payments (ETP)) are 90% of your 2019 tax liability
2) if WH + ETP = 100% of your 2018 tax (110%, if 2018 adjusted gross income over $150,000)
3) tax due after subtracting WH only is less than $1,000
4) was a US citizen or resident and had no tax liability for 2018 on a return that covered the full year.
depending on when you realized your capital gains, you may me apple to reduce any penalty by using the annualized installment method. this method doesn't help if most of your income (stock sales and all other income) was realized early in the year.
estimated taxes can be paid directly using the IRS website. estimates were due, if required, 4/15, 6/15, 9/15 and the next is due 1/15/20 but can be paid now.
it also depends on whether the gains were long term or short term. Long term gains and qualified dividends are taxed at a lower rate.
you can use Turbotax's taxcaster to estimate where you stand
the online versions of TT don't let you see the forms until you pay and you have to avoid clicking the file button. - print to pdf and then review it or buy a desktop version where you can do as many scenarios as you like and not have to pay for each one to see reults.
https://turbotax.intuit.com/tax-tools/calculators/taxcaster
if a broker has your social security number they are not required to withhold.
if they don't 20% withholding but then you have an issue claiming withholding becuase there is no SSN on 1099
