Add random upwards force in a range to object

 private Rigidbody targetRb;

 private float minSpeed = 12;
    private float maxSpeed = 16;

 void Start()
    {
        targetRb = GetComponent<Rigidbody>();
        targetRb.AddForce(RandomForce(), ForceMode.Impulse);

    }

Vector3 RandomForce()
    {
        return Vector3.up * Random.Range(minSpeed, maxSpeed);
    }