Scattering of electrons by localized spins is the ultimate process enabling detection and control of the magnetic state of a spin-doped material. At the molecular scale, scattering is mediated by the orbitals hosting the spin. Here we report the selective excitation of a molecular spin by tunneling through different molecular orbitals. Spatially resolved tunneling spectra on iron-porphyrins reveal that the inelastic spin excitation extends beyond the iron site, changing shape and symmetry along the molecule. Combining density functional theory simulations with a phenomenological scattering model, we show that the extension and lineshape of the inelastic signal are due to excitation pathways assisted by different frontier orbitals. By selecting the intramolecular site for electron injection, the relative weight of iron and pyrrole orbitals in the tunneling process is modified. Thus, the excitation mechanism, reflected by its spectral lineshape, depends on the degree of localization and energy alignment of the chosen molecular orbital.