49 CFR § 385.303T - How does a motor carrier register with the FMCSA?
A motor carrier may contact the FMCSA by internet (www.fmcsa.dot.gov); or Washington, DC headquarters by mail at, Federal Motor Carrier Safety Administration, 1200 New Jersey Ave. SE., Washington, DC 20590-0001; fax 202-366-3477; or telephone 1-800-832-5660, and request the application materials for a new entrant motor carrier. Forms can also be downloaded from https://www.fmcsa.dot.gov/registration/registration-forms. A motor carrier which does not already have a USDOT number must apply online via the Unified Registration System (URS) at www.fmcsa.dot.gov/urs.