Jumping and maintaining current velocity

Hi, I'm trying to make a simple first person controls, I want the movement direction you're moving when you jump to stay the same while in the air. Right now I am able to completely control my character in the air with direction and speed, what would be the easiest way to go about it.

Here is my code:

 void Update () {
         //camera rotation Left and Right
         float rotLeftRight = Input.GetAxis("Mouse X") * mouseSensitivity;
         transform.Rotate(0, rotLeftRight, 0);
         //camera rotation up and down with range
         verticalRotation -= Input.GetAxis("Mouse Y") * mouseSensitivity;
         verticalRotation = Mathf.Clamp(verticalRotation, -upDownRange, upDownRange);
 
         Camera.main.transform.localRotation = Quaternion.Euler(verticalRotation,0,0);
 
         //movement
         float forwardSpeed = Input.GetAxis("Vertical") * movementSpeed;
         float sideSpeed = Input.GetAxis("Horizontal") * movementSpeed;
         Vector3 speed = new Vector3 (sideSpeed, verticalVelocity, forwardSpeed);
         //movement rotates with camera
         speed = transform.rotation * speed;
 
         //gravity
         verticalVelocity += Physics.gravity.y * Time.deltaTime;
 
         //jump
         if(Input.GetButtonDown("Jump") && characterController.isGrounded){
             verticalVelocity = jumpSpeed;
         }
         characterController.Move(speed * Time.deltaTime);
 
 }