As far as I'm aware, you can't define a custom trendline.

There are curve fitting software programs available that do that kind of thing. I think most come with a free trial if you just need it for a one off.

The alternative would be to use the solver in excel to do the curve fitting, then plot that on the graph if neccessary, for example:
A B C D E
1 x1 y1 A1^E1 abs(C1-B1) 2
2 x2 y2 A2^E1 abs(C2-B2) sum(D1:D4)
3 x3 y3 A3^E1 abs(C3-B3)
4 x4 y4 A4^E1 abs(C4-B4)

Then set the solver to set E2 to 0 by changing E1, it won't be very good, but then if the excel ax^b doesn't give you a value close to 1 for a anyway I doubt the curve fitting will be very close in general!