Can I elect Mark to Market accounting under section 475 for the year 2019 after I have e-filed my 2018 Tax Return ? If Yes, whom should I send the election mail to ?

You have until April 15, 2019 to file a section 475(f) election for tax year. 

Please read this IRS document: https://www.irs.gov/taxtopics/tc429

You can send the election to the IRS at the address where you mail your return. Choose the without payment address.

For addresses, please see: https://www.irs.gov/filing/where-to-file-paper-tax-returns-with-or-without-a-payment

you're supposed to attach the election statement to your tax return, which means paper filing.
or to form 4868. by April 15th
if I were in your situation I'd file a paper 4868 and attach the election, even if you have already e-Filed.

Thank you for your very informative email.  Do you know whether TurboTax supports the MTM accounting method for trading after the MTM election is made?  I made the election for the 2018 tax year outside of TurboTax but would like to return to TurboTax this year.  Thank you for your help!

Use TurboTax Deluxe CD/download to report Professional Trader Status.

there's no special computations to be done for the accounting.

On Schedule C you can put accounting method other : Mark-to-Market.
I suspect this is optional since it won't change anything.