When third-party defendant signed and submitted the winning bid and was awarded the contract, it manifested assent to the terms of third-party plaintiffs' offer as set forth in the Project Specifications and the Empire State Building Standards, Specifications and Guidelines, including provisions for indemnification and procurement of insurance. Since the parties entered into this written indemnification agreement prior to the date of plaintiff's accident, the indemnification claims against plaintiffs' employer are not precluded by Workers' Compensation Law § 11. That employer never contested the alleged failure to procure insurance on behalf of third-party plaintiffs.