By applying an appropriate voltage between an STM tip and a metallic substrate, it is possible to induce highly localized electrostatic fields. In this paper, it is shown that the apex of the STM probe, responsible for the resolution, confines an electric field of small lateral extension inside the junction. The change in the potential energy of an adsorbate submitted to such a field is calculated with a self-consistent scheme. A microscopic description of both the tip-apex and the adsorbates is used and the correlations between each polarizable centre are accounted for with a discretized Lippmann-Schwinger equation. Several applications show that our real space approach is extremely attractive for studying electrostatic field distributions in low symmetry systems. Field induced manipulation processes of C-60 molecules are discussed in this context.