Your questions about SoFi Hong Kong are answered here

Yes, margin clients can trade as usual during the pre-market and after-hours sessions. You can also see your real time margin status from the app.

If at any time a margin call does occur (including but not limited to the Core Trading Session and Extended Hours Trading Sessions), SoFi Hong Kong may in its sole discretion, without giving you prior notice sell or otherwise dispose all or part of your stocks and/or close out all or part of your open orders.