Good news, SBI finally figured out they should make that page linkable, and did, here (I think you need to be logged in already for it to work):
https://site0.sbisec.co.jp/marble/trade ... rveList.do
Click 設定変更 for the existing Tsumitate payment you want to make a bonus payment for
決済方法 has to be 現金, you can't set up a bonus for a Credit Card tsumitate (don't do this if you are paying by card already)
In 預り区分 make sure it says NISA（つみたて投資枠）
At the bottom for ボーナス月の積立設定 choose 設定する
Then you can specify the bonus amount and date.
If you are paying by card, or aren't using the Tsumitate portion yet, you'll need to set up a *new* cash tsumitate order, which will buy additional funds every month, then set up the bonus payment on that cash tsumitate. Again, make sure it says NISA（つみたて投資枠）. If your card payment is already maxing out the allowance, you'll need to reduce the monthly card amount first, to give you room to set up the new cash order.
