C API: How to get a seektable for very long files?

Hello,

I'm using flac to compress s signal data during capture. The sample rate
is almost a thousands time higher compared to audio (40 MHz), resulting
in a lot of data very quickly.
I'm using the C API in my capturing application (mostly copy&paste
directly from the example) and it works so far, but unfortunately for
longer captures there is no seeking information. How can I tell flac to
write seeking information? I assume it's not reserving enough space for
the seek table (as a 2 hour capture at 40 MHz is almost 3 month at 40kHz
(flac samplerate field is limited, so I set 40kHz)) and it won't rewrite
a file that is several hundred GBs.
I cannot find any clues in the documentation how to write a
seektable/reserve space for it.
Can someone help me out?

Best regards
Stefan