How do I let the IRS know that I am filing from out of the country?

It's automatic.  Assumedly you will let them know by claiming the foreign earned income credit.  But if you aren't claiming that then you can file form 8854 to let the IRS know that you are an expatriate.  You can file it with your return although TurboTax does not support that form so you have to print and mail your return in order to use it.

@tbernard1958 

When you are out of the country on April 15th, if you use the automatic 2-month extension, you must attach a statement to your return explaining which of the two situations qualify you for the extension.

Note: not all tax software provides a space for such an explanation thereby allowing e-File.

@tbernard1958