Why?   Are you a paid tax preparer?   If so you cannot use TurboTax to prepare returns for clients.   An MEF number is used by paid professional tax preparers.

https://www.irs.gov/e-file-providers/8849-mef-providers

Or....are you really asking about using ID.me so that you can get the monthly child tax credit payments?

https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-t...

Every e-filed tax return has an MeF Submission ID (SID), even if you filed it yourself with TurboTax and did not use a paid preparer. To get your Submission ID you have to call TurboTax support. Go to the following link for instructions to get a phone number to call, during the hours that support is open. The hours are shown at the link.

What is the TurboTax phone number?

If they don't recognize the term Submission ID, tell them it's the "DCN / MeF Submission ID."

It says on the TurboTax link I was sent that if my refund cannot be found that TurboTax will help either the MEF

Dear Sir,

                I need my MEF number (Modern E-File Number) for 2021

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@jwdensmore wrote:

Dear Sir,

                I need my MEF number (Modern E-File Number) for 2021

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Contact TurboTax support.

See this TurboTax support FAQ for contacting support - https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632

Support can also be reached by messaging them on these pages https://www.facebook.com/turbotax/ and https://twitter.com/TeamTurboTax