"saying that my employer for a 'lock-in letter'"
There are some words missing in that sentence, so it's not clear what the notice says, what the status is, or where you are in the process. If the notice that you got is telling you to submit a new W-4 to your employer, go ahead and do that. The new W-4 form for 2020 and later does not use allowances. Use the Tax Withholding Estimator on the IRS web site to figure out what to put on the new W-4. When you get your first paycheck based on the new W-4, double check to make sure that your employer entered the new W-4 information correctly.
If the IRS already issued a lock-in letter, call the number that the IRS gave you.
