Do not think too much of your references in the beginning. Think about it like this. Most startups do not have any references to brag with when they start out but are still able to find partners.
However what you should think about is your USP. What is it that makes your cashback reward app unique and why should anyone partner up with you, when there are already well established cashback apps with huge audiences.
When you figure that out, come up with an elevator pitch as you would for an investor, where you clearly detail what makes you unique compared to others, what the advantage would be for your partner and how you plan to scale your audience.
The last part is also important. Many people always wonder how entrepreneurs come up with any numbers for their future revenue, when they are just starting out. Well they are making them up in sense and investors know it. The questions is not do you hit the exact numbers you have presented in your first pitch or come even close to it but do you have an exact plan and a grasp of the numbers and marketing dynamics. How else would someone trust you and partner up with you?
Last but not least make it as easy and painless for them to join your program. Offer them a sponsored positioning for a year if they try you out or a blast out to your future e-mail list/ push notifications. Anything that makes it a you can't lose deal.
When it comes to approaching your partners make sure you know exactly who's your ideal partner. Then do not start with the biggest player. They are more conservative and will most likely join you later when you have other partners similar to them. However mid-size companies are often more flexible and keen to try something new. Go to LinkedIn and find the right people from marketing or business development. Connect with them. See if you have any common friends and try to get a warm introduction. Otherwise your objective should be to schedule an initial call where you can pitch your idea in ideally 5 to 10 minutes as you will not get more time. When they are interested you schedule a follow up immediately.
If you need more help it would be my pleasure.
Best of luck.