You can do this by splitting your expenses into different categories.
First, create a custom expense category called HST by going to Settings > Expense Categories
Next, Go to your Transactions page and locate the expense you'd like to split. (We suggest using the Expenses Only under Filter by Types so you have less transactions to sort through)
Open the expense to edit it, and click the Split button at the bottom of the screen. This will split the expense into two. (Click Split again to split it a third time, and so forth)
Type in the amount of the first split (or the base cost of the item)
Type in the amount of the second split (or the HST)
Save your changes!