public class test : MonoBehaviour
{
 [SerializeField] private Camera camera;
 private Vector3 _diagonalVector;
 public Vector3 wantedVector;

 private void Update()
 {
 var distance = 1.0f;
 var frustumHeight = 2.0f * distance * Mathf.Tan(camera.fieldOfView * 0.5f * Mathf.Deg2Rad);
 var frustumWidth = frustumHeight * camera.aspect;
_diagonalVector = new Vector3(
 frustumWidth/2,
 frustumHeight/2,
 distance);
var rotation = Quaternion.FromToRotation(Vector3.forward, camera.transform.forward);
 wantedVector = rotation * _diagonalVector;
 }

 private void OnDrawGizmos()
 {
 Gizmos.color = Color.red;
 Gizmos.DrawLine(camera.transform.position, camera.transform.position + wantedVector);
 }
}