Are you trying to anchor both your ring finger and pinky or just your pinky?
Some people just anchor their ring finger so if that feels more comfortable that’s also a possibility.
If you’re just planting your pinky you could try adding your ring finger and see if that helps stabilize your hand.
Otherwise it may just be more practice is required. Start slow and if you feel your pinky pop off the head stop and plant it again and start over.
If you have any other questions let me know.