If the marriage license asks for your Denomination, write "Non-Denominational", unless the marriage was celebrated in accordance with a specific denominational rite, which you should specify.
By performing marriage as an AMM minister, you are acting as a representative of American Marriage Ministries, a non-denominational and interfaith church. For this reason "Non-Denominational" is usually the most appropriate response.
If your ceremony was celebrated with the tradition of a specific denomination, you should specify this (eg. Baptist). Additionally, there are a few counties that will not allow ministers to specify "Non-Denominational", if you are informed of this by your local county clerk, write "American Marriage Ministries".