  We develop a method to calculate helicity amplitudes of an arbitrary
tree-level process in Feynman-Diagram (FD) gauge for an arbitrary gauge model
with MadGraph5_aMC@NLO. We start from the 't Hooft-Feynman gauge Lagrangian in
FeynRules and generate scattering amplitudes by identifying the Goldstone boson
as the $5^{\rm th}$ component of each weak boson. All the vertices of the
5-component weak bosons are then created automatically by assembling the
relevant weak boson and Goldstone boson vertices in the Feynman gauge. The
5-component weak boson vertices are then connected by the $5\times5$ matrix
propagator in the FD gauge. As a demonstration of the method we calculate the
cross section for the process $\mu^-\mu^+\to\nu_\mu\bar{\nu}_\mu t\bar{t}H$
with complex top Yukawa coupling, which can be obtained by adding a gauge
invariant dimension-6 operator to the Standard Model (SM) Lagrangian. The FD
gauge and the unitary (U) gauge amplitudes give exactly the same cross section,
and subtle gauge theory cancellation among diagrams in the U gauge at high
energies is absent in the FD gauge, as has been observed for various SM
processes. In addition, we find that the total cross sections at high energies
are dominated by a single, or a set of non-vanishing Feynman amplitudes with
the higher dimensional vertices in the FD gauge.
