By when must I buy to receive new shares from a bonus issue?

Bonus shares are allotted in proportion to the number of shares held by shareholders as of the record date for the allotment of new shares.

Because settlement takes place on T+2, you must buy at least 2 business days before the record date to receive the new shares, and the stock goes ex-rights 1 business day before the record date. The higher the bonus issue ratio, the larger the price adjustment on the ex-rights date.