Line Direction

To get the proper direction for shooting the ball you need to subtract the end position of the line from the starting point of the line and normalize the vector to get the x and y directions. You then keep updating this value as you drag your mouse around. I made a prototype of a golf shooting mechanic so you can see the entire thing put together.

This script is placed on the game object that has your LineRenderer:

 using UnityEngine;
 
 public class ShotLine : MonoBehaviour
 {
     public Rigidbody2D ball = null;
     public float force = 100.0f;
     public bool isStartAtMouse = false;
     public bool isLineVisible = false;
 
     private LineRenderer line = null;
     private Vector2 startPosition = Vector2.zero;
     private Vector2 endPosition = Vector2.zero;
     private Vector2 direction = Vector2.zero;
     private float power = 0.0f;
     private bool isShot = false;
 
     private void Awake()
     {
         line = GetComponent<LineRenderer>();
     }
 
     private void Update()
     {
         if (Input.GetMouseButtonDown(0))
         {
             if (isStartAtMouse == true)
             {
                 startPosition = Camera.main.ScreenToWorldPoint(Input.mousePosition);
                 line.SetPosition(0, startPosition);
             }
             else
             {
                 startPosition = ball.position;
                 line.SetPosition(0, startPosition);
             }
         }
 
         if (Input.GetMouseButton(0))
         {
             endPosition = Camera.main.ScreenToWorldPoint(Input.mousePosition);
             line.SetPosition(1, endPosition);
             direction = (endPosition - startPosition).normalized;
             power = Vector2.Distance(startPosition, endPosition);
         }
 
         if (Input.GetMouseButtonUp(0))
         {
             if (isLineVisible == false)
             {
                 line.SetPosition(0, Vector2.zero);
                 line.SetPosition(1, Vector2.zero);
             }
 
             isShot = true;
         }
     }
 
     private void FixedUpdate()
     {
         if (isShot == true)
         {
             ball.AddForce(direction * (power * force));
             isShot = false;
         }
     }
 }
 

There's some extra stuff in there, like if you want to toggle the visualization of the shot staying on screen or not. Also if you want to start the shot from anywhere on screen and not have it locked to the ball while maintaining the proper shooting direction. This demonstrates that it doesn't matter where you click and drag on the screen so long as you normalize the two vectors.

As for the ball game object itself it just has a Rigidbody2D with gravity scale set to 0 and a Circle Collider2D.

And here's the result of this code: This is with the IsStartAtMouse option set to true:

I hope this helps! Good luck!