Visit https://www.irs.gov/pub/irs-pdf/fw4r.pdf to download the IRS form (3 pages) W-4R Withholding Certificate for Nonperiodic Payments and Eligible Rollover Distributions.
You may mail this form to TRS at:
Teachers’ Retirement System
PO Box 19253
Springfield IL 62794-9253
Or you may fax it to: (217) 753-0964.