To configure the trust for your AWS Managed Services (AMS) account, follow this MicroSoft AD article
Create a one-way, incoming, forest trust for one side of the trust
Open the Start -> Administrative Tools -> Active Directory Domains and Trusts dialog. Right-click the domain node for the domain that you want to establish a trust with, and then click Properties -> Trusts -> New Trust to open the New Trust Wizard. Enter the domain name provided to you by AMS for the Trust Name and press Next.
Under Trust Type, select Forest Trust. Press Next.
Under Direction of Trust, select One-way: incoming. Press Next.
Under Sides of Trust, select This domain only. Press Next.
Under Trust Password, type a password of your choosing. Press Next.
For Trust Selections Completed and Trust Creation Complete, just press Next.
Under Confirm Incoming Trust, select No, do not confirm the incoming trust. Press Next.
Under Completed the New Trust Wizard, select Finish, and then OK to close.
Provide the trust password (contact us via your CSDM's phone number for security reasons). AMS will complete the trust configuration.