Your spouse/partner's net income (Line 23600) is calculated by subtracting social benefit repayments (Line 23500) from their net income before adjustments (Line 23400).
Line 23400 less Line 23500 equals Line 23600. So, if your spouse didn't have any social benefit repayments, then Line 23400 is the same as Line 23600.
You can put in an estimate for now and then put the correct number before you file.