So since the Sega Genesis uses an RF cable instead of an AV cable, how do I send this signal to my capture card?

swap the cable

https://www.amazon.com/Mizar-Replacement-Sega-Genesis-Standard-Cable/dp/B002FY0CM0

The model 2 and 3 Genesis have A/V out over RCA. If you're stuck with a model 1, then get an RF to RCA converter.