In general, you'll need to find t closest to your search point. There are a number of ways you can do this, there's a chapter on finding the nearest point on the bezier curve. In my experience, digitizing the bezier curve is an acceptable method. You can also try recursively subdividing the curve, see if you point is in the convex hull of the control points, and checking is the control points are close enough to a linear line segment and find the nearest point on the line segment, using linear interpolation and keeping track of the subdivision level, you'll be able to find t.