The Jan2024 list is out (they publish weeks before it is official to use) - Daxi has a C code but not a specific J code. So I assume they will have a J code April 2024. C codes are more for internal bookkeeping but are not acceptable for billing.
Providers can use a Miscellaneous code (J3490) but would be accepting some risk if not explicitly pre-auth'd with that code. We go through these hassles every time a new MS or other neuro drug comes out - most recently ublituximab (FDA approved Jan 2023, J code effective July 2023) and lecanemab. Some insurers will allow J3490 miscellaneous codes before the J code is authorized (medicare has for the MS drugs but we are always cautious until the J code is valid) but private insurers are more variable and often do not.