Squat Stretch is my favorite for preparing my body for squatting. Simply sitting in a squat and relaxing and letting it get deeper and deeper. Push out on the knees with your elbows to stretch groin. Also, arch back and keep chest up/out to really feel the hip flexors stretch (which is likely contributing to butt wink).
I don't advise stretching immediately before your workout, but if you do this after your workout and maybe once more during the day you should feel more mobile during your workout.
Also, a neat trick I learned from a Bret Contreras video, is to squeeze the glutes hard at the bottom (takes practice). First, try it while in the squat stretch. Sometimes butt wink is caused not by immobility, but by the femur coming into contact with the pelvis, so the lumbar region has to flex to allow continued range of motion. Contracting the glutes pulls the head of the femur back away from the pelvis just enough to allow a few more degrees of motion.
James 1:16-17 ESV
Do not be deceived, my beloved brothers. Every good gift and every perfect gift is from above, coming down from the Father of lights
With God's help...Mens sana in corpore sano