Your questions about SoFi Hong Kong are answered here

Yes. No later than 3 business days before the 15th of each month, you need to edit the amount of your “Monthly Contribution”.

In order to do so:

Under Auto Invest, select your investment portfolio Under “Monthly Contribution”, tap onto your current US$ amount. Ensure “Activate Monthly Direct Debit” is toggled on. Enter an amount into either the “USD” or “HKD” field. Hit “Save” at the bottom of the page.