https://vk.com/doc229493618_471690088
Hope, this worked :\I think the gif issue is that it was added as an attachment. I think if you add it via this button it should play.
the video link wasn't playing for me either so i'm not exactly sure what the issue is. but this is how I would squat with very little fuss. detach the feet from the rig for the frames your character is squatting; this ensures that the feet appear to be firmly planted on the ground and don't move around. the rig's root should be around the waist or torso. lower the rig in animate mode and then use ik to adjust the placement of the legs and torso into the final squatting position. reattach and position the feet when needed. a tip, adjusting the easing of the calves to be more steep will keep your ankles in place.
IK in opentoonz can be a bit wonky and gets harder to control as rigs begin to incorporate "peg" columns.