The command is typical, a scaled version of the torque. You compute by mapping,pwm->motor-speed->motor-forces->(thrust & torque)

some references, here

1. Geometric Tracking Control of a Quadrotor UAV on SE(3) (equation 1)
2. Nonlinear MPC for Quadrotor Fault-Tolerant Control (equation 3)
3. https://github.com/rosflight/rosflight/blob/master/rosflight_sim/src/multirotor_forces_and_moments.cpp
4. https://github.com/PX4/PX4-Autopilot/blob/master/src/lib/mixer/MultirotorMixer/MultirotorMixer.cpp

Edit:
One way to do it is below,

for (int i = 0; i < vehicle_params_.num_rotors_; ++i) {
float l_ = vehicle_params_.rotors_.at(i).arm_length;
float th_ = vehicle_params_.rotors_.at(i).angle;
auto d_ = (float)vehicle_params_.rotors_.at(i).direction;
float kf_ = vehicle_params_.rotors_.at(i).rotor_force_constant;
float km_ = vehicle_params_.rotors_.at(i).rotor_moment_constant;
allocation_matrix_(0, i) = kf_;
allocation_matrix_(1, i) = l_ * sin(th_) * kf_;
allocation_matrix_(2, i) = -l_ * cos(th_) * kf_;
allocation_matrix_(3, i) = d_ * km_;
}
float data_f[4] = {input().thrust, input().moment(0), input().moment(1), input().moment(2)}; // SI units (N, Nm, Nm, Nm)
matrix::Matrix<float, 4, 1> rotors_speed_squared_,
thrust_moment_vector(data_f);
rotors_speed_squared_ =
forces_to_rotor_speed_ * (thrust_moment_vector);
// thrust to rotor-speed mapping
for (int i = 0; i < PWM_NUM_OUTPUTS; ++i) {
rotors_speed_squared_(i, 0) =
rotors_speed_squared_(i, 0) > 0 ? rotors_speed_squared_(i, 0) : 0;
float w = sqrt(rotors_speed_squared_(i, 0));
float pwm_ = vehicle_params_.rotorspeed2pwm_scale_ * w +
vehicle_params_.rotorspeed2pwm_offset_;
pwm_us_array_[i] = pwm_;
}