Get Rigidbody2D component in script (c#)

You get a reference to it.

Since you have the PlayerMovement component on the same game object, GetComponent is your friend.

If you are able to store the reference in a field(variable) then you can reuse it has the script is called over and over again.

example in start(c#):

 using System;
 using UnityEngine;
 
 public class HeadBob : MonoBehaviour
 {
     private Rigidbody2D myScriptsRigidbody2D;
     
     void Start()
     {
         myScriptsRigidbody2D = GetComponent<Rigidbody2D>();
     }
     
     void Update()
     {
         myScriptsRigidbody2D.AddForce(new Vector3(6, 8, 1)); // just an example.
     }
 }

example in start(java/unityscript)

 private myScriptsRigidbody2D : Rigidbody2D;
 
 function Start()
 {
     myScriptsRigidbody2D = GetComponent.<Rigidbody2D>();
 }
 
 function Update()
 {
     myScriptsRigidbody2D.AddForce(Vector3(6, 8, 1)) // just an example.
 }