,How do i make a D-Pad that functions on AddForce??

I am having trouble making a on-screen D-Pad that functions on AddForce. I have looked up other tutorials but they work on Axis. The reason i need AddForce for my game is because i am making a 3D mobile game where you roll a sphere to the finish. If i were to use axis, the ball wouldnt roll, it would just go forward which is just ugly. I need the D-Pad to add force to the player in the direction of the joystick. The ball already rolls on WASD, heres the code for it:

using UnityEngine;

public class PlayerMovement : MonoBehaviour{

public Rigidbody rb;

public float forwardForce = 2000f; public float sidewaysForce = 500f;

// Update is called once per frame void FixedUpdate() {

 if ( Input.GetKey("d") )
 {
     rb.AddForce(sidewaysForce * Time.deltaTime, 0, 0);
 }

 if (Input.GetKey("a"))
 {
     rb.AddForce(-sidewaysForce * Time.deltaTime, 0, 0);
 }

 if (Input.GetKey("w") )
 {
     rb.AddForce(0, 0, sidewaysForce * Time.deltaTime);
 }

 if (Input.GetKey("s"))
 {
     rb.AddForce(0, 0, -sidewaysForce * Time.deltaTime);
 }

} }

Any help is appreciated, even some code wouldn't hurt as i'm stuck on this for 2 weeks. Thank you very much in advance.