The study of resonance form factors in lattice QCD is a challenging endeavor. Namely, the infinite-volume limit, $L \rightarrow \infty$, is not well defined in the matrix element (here, $L $ is the spacial extension of a rectangular lattice). This irregular behavior persists even after multiplying each external leg with the pertinent Lellouch-Lüscher factor and stems from the so-called triangle diagram.
In this talk, I shall discuss a novel method to tackle this problem in which the difficulty, related to the presence of the triangle diagram, never emerges. The approach is based on the study of two-particle scattering in a static, spatially periodic external field by using a generalization of the Lüscher method in the presence of such a field. In addition, I shall demonstrate that the resonance form factor in the Breit frame is given by the derivative of a resonance pole position in the complex plane with respect to the coupling constant of the external field. This result is a generalization of the well-known Feynman-Hellmann theorem for the form factor of a stable particle.