  Polaritons in polar biaxial crystals with extreme anisotropy offer a
promising route to manipulate nanoscale light-matter interactions. The
dynamical modulation of their dispersion is great significance for future
integrated nano-optics but remains challenging. Here, we report a
momentum-directed strategy, a coupling between the modes with extra momentum
supported by the interface and in-plane hyperbolic polaritons, to tailor
topological transitions of anisotropic polaritons in biaxial crystals. We
experimentally demonstrate such tailored polaritons at the interface of
heterostructures between graphene and {\alpha}-phase molybdenum trioxide
({\alpha}-MoO3). The interlayer coupling can be electrically modulated by
changing the Fermi level in graphene, enabling a dynamic topological
transition. More interestingly, we found that the topological transition occurs
at a constant Fermi level when tuning the thickness of {\alpha}-MoO3. The
momentum-directed strategy implemented by interface engineering offers new
insights for optical topological transitions, which may shed new light for
programmable polaritonics, energy transfer and neuromorphic photonics.
