There is no portal for that. You would open a TurboTax account and file a regular tax return. Instead of doing that, wait a few more days for the IRS to open up a portal that will allow you to enter your direct deposit information without the need to prepare and file a complete tax return. Here is a link to follow: https://www.irs.gov/newsroom/economic-impact-payments-what-you-need-to-know
