How to tween a variable using iTween

No, it's not possible with one line of code. Tweening a value in iTween.ValueTo() involves using the use of an onupdate callback. Here is an example:

 int exampleInt = 0;
     
 iTween.ValueTo( gameObject, iTween.Hash(
     "from", exampleInt,
     "to", 100,
     "time", 1f,
     "onupdatetarget", gameObject,
     "onupdate", "tweenOnUpdateCallBack",
     "easetype", iTween.EaseType.easeOutQuad
     )
 );
     
 void tweenOnUpdateCallBack( int newValue )
 {
     exampleInt = newValue;
     Debug.Log( exampleInt );
 }

As a note, it may be possible in one line with iTween's native extensions for Unity, which you have to purchase. It allows you to do something like:

 gameObject.ValueTo(...);

instead of having to call the Type Reference, such as iTween.ValueTo. It probably still needs an onupdate callback, I would assume.