As stated in the instructions for the Form 433-D, return Part 1 to the IRS. Part 2 is your copy you keep for your records.
Read the Terms of this agreement. You are authorizing the IRS to debit your bank account for the amount indicated until the debt has been satisfied. This is a legal document.
IRS Form 433-D - https://www.irs.gov/pub/irs-pdf/f433d.pdf
When you’ve completed this agreement form, please sign and date it. Then, return Part 1 to IRS at the address on the letter
that came with it or the address shown in the “For assistance” box on the front of the form.