That’s not how this works, the only time you need to subscribe for drops is if there is a specific campaign that usually requires you to gift two subs for the reward, otherwise it’s just twitch for x amount of time and get the reward.
Also just a friendly reminder if you’re like me and just want to get the reward without watching you can just set the video quality to like 480p and then mute the volume using the mute button on the tab in your browser, this part is important because if you just lower their volume to zero on twitch itself it won’t count your view time towards the drop reward.