TLDR: You can add a bonus to your income by selecting 'Yes' under other savings when adding the 'Savings' source, then entering your bonus details.
Showing your income
If you receive a bonus as part of your income, this can be added when adding the 'Savings' source, as part of your source of funds task.
Adding a bonus
When adding the 'Savings' source, you will be asked if you have any other savings, such as receiving any rental income, dividends or benefit income. Please select 'Yes' and you will then be able to enter the details of your bonus.