To access the 100% FY capital allowance the company simply has to have the car as an asset on its balance sheet, i.e. it has to be bearing the risk of the unknown future depreciation.
That could be by buying it outright, but it could equally be by taking out a secured loan against it, or through some finance lease arrangements. The key is that the company must be bearing the risk of the depreciation, so it must either already own the car, or it must have a finance arrangement where at the end there is a (non-optional) final payment resulting in ownership of the car.
I would think that Tesla's approved finance options would include one which meets these requirements, i.e. where you could get both the 100% FY allowance and also their residual guarantee.
But I'm not an accountant, and I haven't looked at Tesla's finance options since they didn't exist when I bought my car.