OnItemSaving.cs

public void OnItemSaving(object sender, EventArgs args)
{
	var eventArgs = args as SitecoreEventArgs;

	var updatedItem = eventArgs.Parameters[0] as Item;

	if (this.ShouldSkipItem(updatedItem))
	{
		return;
	}

	var itemChanges = eventArgs.Parameters[1] as ItemChanges;

	if (itemChanges.FieldChanges.Contains(Constant.FieldId.ApplicationFieldId)) //Application Field ID
	{
		var applicationLinkNewValue = itemChanges.FieldChanges[Constant.FieldId.ApplicationFieldId].Value;

		if (!applicationLinkNewValue.IsNullOrEmpty())
		{
			Match match = Regex.Match(applicationLinkNewValue, Constant.Pattern.IdPattern);
			
			if (match.Success)
			{
				var idValue = match.Groups[1].Value;

				var applicationItem = this.CoreDatabase.GetItem(new ID(idValue));

				if(applicationItem != null)
				{
					var path = applicationItem.Paths.ContentPath;

					applicationLinkNewValue = Regex.Replace(applicationLinkNewValue, Constant.Pattern.IdPattern, $"url=\"{path}\"");

					itemChanges.SetFieldValue(updatedItem.Fields[Constant.FieldId.ApplicationFieldId], applicationLinkNewValue);
				}
			}                    
		}
	}
}