I don't think you can do this with a Geometry Service or the API's geometryEngine tools. You might need to iterate through the points in the polyline path, testing the distance from the start to the path point each time, until you reach the path point just beyond the distance you require. Then you know the required distance point is between the last path point (n) and the path point just before the required distance (n-1), of which you know both points' distance from the start as well as the distance between path point n and path point n-1. Now you can calculate the coordinate of your required point with basic Pythagoras theorem.
