Step 1: import the model and go in the place where you want to edit it.
Step 2: In Edit Mode, select a loop around the place you want to pose, in this case it's a leg.
Step 3: Right click, mark seam!
Step 4: how the marked seam edges should look like!
Step 5: Enter sculpt mode, Initialize face-sets from up-left part of viewport, By UV seams.
Step 6: How it should look.
Step 7: Select the Pose Brush, Up in the viewport , Brush Settings, change Rotation origins to face sets.
Step 8: Pose!