The Mark can be used both before and after a workout, or any other activity that you enjoy doing. However, if you only had to pick one or the other, we'd encourage you to use the Mark before a workout.
Using the Mark Before a Workout
Releasing tension in your hip flexors before you work out can help you improve the range of motion of your hips along with the alignment of your pelvis, both of which can help you feel better during your workout. This can have the effect of allowing you to strengthen and stabilize around this better alignment, leading to longer-lasting improvement with your pain. Many users of the Mark have also reported feeling an increase in performance during their workout when releasing their hip flexors beforehand.
Using the Mark After a Workout
Similarly, the Mark can also be used as part of a workout recovery routine after you exercise. This may prevent the hip flexors from tightening up, as these muscles may be used a lot in any given activity. Helping these muscles relax and recover after your workout (or later in the same day that you worked out) may help you feel strong and keep your psoas and iliacus happier more consistently day after day.
Be Proactive, Not Reactive
Ultimately, the Mark can be used in more of a preventative way pre-workout to help get your body into better alignment. As it relates to using the tool post-workout, ideally it would be used to help with recovery (in a good way) following a tough workout, and not as something to "undo" the pain that was caused by pushing through a workout while your hips were tight and/or out of alignment.