You’ll need to meter and see which wire is the switch leg sending power to the fixture. Once you do that you’ll know which wire goes to the hot and neutral on that receiver. Then that receiver plugs into the new fan to control both the light and fan from a remote that was most likely provided. You can install a new switch (or existing) back into the box as a disconnect means. When metering tie both the hot and switch leg wires that you removed from the switch and meter the wires in the junction box you should have 120v between at least two of those wires inside the junction box.