Make content worth subscribing for. Literally the only way.
Ask yourself, why do YOU subscribe to anyone? Usually it's because the creator you subscribe to, makes content you enjoy watching.
Make content others enjoy watching, put appropriate, not overbearing calls to action, a quick reminder of what value you provide and how often you provide it, leave it at that, and make the rest of the video so good that the audience wants to come back again and again.
Also, you won't be making adsense the moment you hit 1k, you don't get your first payout until you hit $100, and some channels can take months to get a payout. If you make $20 a month in adsense, you're going to have to wait 5 months before ever getting paid.
So with all that in mind, why is 1k important to you other than the milestone, and let's be real, we all enjoy hitting milestones.