65mm threads are not a standard thread size for lens filters. This means you'll have a very difficult time finding a thread-on filter (any filter) for your wide-converter.
There are slide-in filters (e.g. Lee Filters, Cokin, etc.) that slide into an adapter, but again, even the filter holder needs an adapter ring that matches the thread size of the lens and you don't have a standard thread size ... hence no pre-made adapter rings are available.
Lee Filters does offer custom-made adapter rings in any thread size ... but custom adapters are going to be expensive. You may prefer to eliminate the wide-angle converter and go with a lens that offers the focal length you want (and it would have standard thread sizes to accept filters).
It is also possible to get custom-machined step-up adapters for threaded filters offered in standard sizes. E.g. if you want to mount say, a 77mm thread-on filter onto your 65mm thread size, you buy a 65 to 77mm "step-up" ring.
Both step-up rings and slide-in filters have the benefit that you can buy one size filters (large enough to fit the largest lens you own) and then get step-up rings (or adapter rings for slide-in filters) for any smaller lens sizes you own (avoiding the cost of buying a circular polarizing filter in every lens size you might eventually own.)
I offer the "custom" adapters as a solution to your direct question (because you asked how to get a filter to fit wide-converter), but if it were my money, I wouldn't go with the custom solution. I'd get rid of the wide-converter (for a lot of reasons... quality, metering accuracy, focus accuracy/reliability, etc.). They are usually ... not great. Part of the point of owning a camera with detachable lenses is that you can get a lens to fit your needs rather than using one lens and use focal length adapters.