You can apply for FLISP after bond registration, although the application must be sent within 12 months.
Is it recommended you apply after bond registration? It’s probably better to apply immediately after having a home loan approved so you can use the subsidy to pay for fees such as bond registration.
Applying even earlier, such as before attempting to obtain a home loan, will enable you to use the subsidy as a deposit. You could also pay it directly into your home loan, reducing the monthly repayments and interest rates.