We agnostic still believe in a higher force but reject organized religion.
I consider myself to be agnostic but don’t feel that it matches with your description here. As an agnostic, I feel that we as humans have no way of knowing whether a “higher force” exists or not. There may be a supernatural power, it may be God, or it may be something else, or it may not exist at all and life is only a result of our physical world. Unless substantial evidence is provided within my lifetime, I will have to wait until I die to find out the truth. If life simply ends and there is no afterlife, then I won’t give a damn anymore anyway.
So, I don’t feel that there can necessarily be a common prayer among agnostics, to express our common morality. We don’t have a common morality; everyone follows their own intuition of what is right and wrong. I express my morality in the choices I make and the way I treat others on a daily basis.
I can think of a basic mantra for agnostics though: “The only true wisdom is in knowing you know nothing.”