The formulation will be based on the desired quality parameters you want in the finished product, as well as, the process used for the product.
For example, if you use UHT processing you will need some type of process aid to run it through the equipment without creating too much burn on in the equipment. This will be dependent on what type of UHT process technology used such as direct steam or indirect steam, or something else.
It isn't clear if you have a way to manufacture this, are seeking a co-packer, or starting from the ground up. If you are starting from the ground up you need to find someone who is an expert in this area otherwise you can surely lose A LOT of money in the process...mostly just wasting money from the incorrect type of equipment used, wasted batches during R&D to scale up, etc.
If you want an aseptic or extended shelf-life product it is probably best to find a processor that can co-pack it for you under your brand. This processor will also have the expertise and can help with the process itself, and give some guidance on formulation (although they will be hesitant to do it all for you).