In QuantumATK-P-2019.03, the total spin magnetic moment is actually printed out in the MullikenPopulation object.
If you are using an older version of QuantumATK, you should calculated youself the difference between the spin-up and the spin-down components of the Mulliken population. Notice however that in a non-collinear framework, this difference may not correspond to the total magnetic moment.
Best,
Daniele