[Manager Names]
My time at [Company Name] has been a great experience. It has been a pleasure working with you. As you may know I created [ Open source project ] on [Date] and have enjoyed growing the product and user base. Building and supporting successful products is something I enjoy and will be looking for opportunities to do that outside of my current role. [Day 2 weeks from now] will be my last day at [Company]
Best of Luck
Then you can negotiate from a place of power. Don't say anything about your plans other than that you are going to pursue other opportunities. At a minimum they will make you a counter offer, something like 10% pay increase and a new title of "Product Director". It is up to you if you want to negotiate something different like back pay or if you want to try to create a new product from scratch on your own or if you want to find another job.