applying force in any direction

     var moveDirection = Vector3.zero;
     var speed = 1.0;
     if FixedUpdate ()
            {
            moveDirection = new Vector3(0, 0, Input.GetAxis("Vertical")*speed);
            moveDirection = transform.TransformDirection(moveDirection);
            moveDirection *= speed;
            }
     if(Input.GetAxis("Horizontal")  < -.1 )
           {
          transform.Rotate(Vector3.up * -.6);
          particleEmitter.enabled = true;
           }
    if(Input.GetAxis("Horizontal") > .1)
          {
        transform.Rotate(Vector3.up * .6);
        particleEmitter.enabled = true;
          }
          }

when i press up arrow it moves moves forward and when press left and right arrow it turns left and right direction.

when i release the up arrow i want to add a constant force along the direction now the character is facing.

i want to translate(move) the character to certain constant distance after the release of the up arrow.

i want to move through the direction now the character is facing. it should happen after the release of up arrow only.

i want to add force or translate the character the direction now the character is facing after the release of up arrow.

i tried a lot not getting the output.

(Input.GetKeyUp (KeyCode.UpArrow)) but not working for me