Read the theory: blake3-totp.pdf. Implement the construction described there, using the shared secret below, and generate valid 8-digit codes yourself.
Shared secret (Base32): CB2X3YYGRPP4ZDVBUJ4PN5P4OJBRPEVI
To get the flag: submit a correct, currently-valid 8-digit code, then submit a different correct code within 60 seconds (i.e. wait for the next 30-second step and compute again).