I have tried it, the following prompts can make this posture more accurate

Trigger Words 1:

((Cowgirl Position)), 1 girl, leaning back, Middle Splits, Splits, 1 boy, Lying, grabbing ankles, Big penis, vaginal penetration

Trigger Words 2:

((Cowgirl Position)), 1 girl, leaning forward, Middle Splits, M_legs, grabbing own breasts, 1 boy, Lying, grabbing ankles, Lift feets, Big penis, vaginal penetration

Trigger Words 3:

(Cowgirl Position)), 1 girl, leaning back, legs up, 1 boy, Lying, grabbing kneepits, Big penis, vaginal penetration