When your margin funding expires and your margin trading position is still open, the system will automatically reserve the required margin funding at the current prevailing rate available.
When margin funding used for your active margin trading position expires, the platform will always use any available manually reserved margin funding first; and if needed, will fill the remainder automatically with the current prevailing rate available.
If you want more control over the rate and period of the margin funding, you can manually reserve margin funding via the Funding page before your current funding expires.