Unfortunately, you still have to estimate and enter amounts, based on what you know. For example, if you have a separation agreement, you might be able to estimate your ex-spouse’s income from there.
Keep in mind, if you enter amounts that are too low, you might end up claiming benefits that you’re not entitled to, or receiving amounts that are higher than they should be. This might result in the Canada Revenue Agency (CRA) clawing back these benefits, and you having to pay interest charges and penalties.
Why is this information necessary?
Since many family and social benefits (such as the Canada child benefit, GST/HST credit, and working income tax benefit) are based on the net family income from the previous year, a change in your marital status results in a re-calculation of these benefit amounts.
You separated from your spouse in 2018 and received full custody of your two children. At the time of your separation, neither you nor your spouse received the GST/HST credit as your 2017 net family income was too high. However, now that you’re a single parent with two children in your care, the CRA might find that you’re entitled to the GST/HST credit.