All players should have received an in-game mail titled "[Rewards] A Special Thanks from Wuthering Waves Dev Team", which will have the Voucher of Reciprocal Tides Convene Token attached to it. To use this, you need to reach Union Level 8 or unlock the Convene function through the trial in "First Resonance", then you can use the voucher on the Convene screen and select a 5-Star Resonator to claim for free. This voucher is valid until May 22, 2025, so you have plenty of time to meet those minimum requirements.