This paper proposes an algorithm for the automated design of metasurface antennas. From the knowledge of an objective radiation field and of an incident field, it allows to synthesize the required surface impedance on the radiating aperture. A scalar impedance boundary condition is employed in the integral equation formulation of the electromagnetic problem. The approach is entirely numerical, based on an iterative algorithm able to enforce both physical and feasibility constraints. This procedure was applied to the design of a leaky-wave antenna at 30 GHz and a circular metasurface antenna at 17 GHz. The results showed a good performance compared to a reference analytical design.