How should I implement jumping into this script?

When I try to implement jumping into this script, the "playerRb.velocity = playerTf.right xInput velocityMultiplier + playerTf.forward zInput velocityMultiplier" overrides and cancels the added velocity/force. Any help welcome :)

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerMovement : MonoBehaviour
 {
     // Ground Checker
     bool isGrounded;
 
     public Transform groundCheckerTf;
 
     public LayerMask groundLayer;
     
     // Input and forward and sideways movement
     float xInput;
     float zInput;
 
     public Rigidbody playerRb;
     public Transform playerTf;
 
     public float velocityMultiplier;   
 
     // Start is called before the first frame update
     void Start()
     {
         
     }
 
     // Update is called once per frame
     void Update()
     {
         // Ground Checker
         isGrounded = Physics.CheckSphere(groundCheckerTf.position, 0.05f, groundLayer);
 
         // Forward and sideways movement
         xInput = Input.GetAxis("Horizontal");    
         zInput = Input.GetAxis("Vertical");
 
         playerRb.velocity = playerTf.right * xInput * velocityMultiplier + playerTf.forward * zInput * velocityMultiplier;        
     }
 }