The "source" suffix in the filename might imply that a specific frame contains the key.
strings 0h8q1ak2acptoechqhoxq_source.mp4 | grep -i "CTF" (or the specific tag for the competition). 0h8q1ak2acptoechqhoxq_source.mp4
Scour the output for readable text that looks like a password or a URL. The "source" suffix in the filename might imply
Sometimes the flag is simply appended to the end of the file or hidden in the binary data. Tool: strings 0h8q1ak2acptoechqhoxq_source.mp4