In Venezuela, near the border with Colombia, in the region of the mouth of the Catatumbo River where it empties into Lake Maracaibo, there is something similar to a perpetual storm. It is not really a perpetual storm, but a series of storms that forms in the same place and stay in the area. The place has more storms than anywhere else in the world by an overwhelming margin.
Those storms persist for around 140 to 260 nights per year. Each storm lasts for roughly 10 hour a day, producing roughly 280 lightning strikes each hour. This is an average of one lightning strike every 15 seconds for the duration of each individual storm. And of course, there should be times of more intense activity than the average.
Relevant wikipedia source: https://en.wikipedia.org/wiki/Catatumbo_lightning
In order to create a place with a truly perpetual storm in your hypothetical planet, you should optimize/maximize the conditions that make places like Catatumbo real (and put your island there). Unfortunately, the cause of such phenomena is still poorly understood. It seems certain that near-equator latitude, oceanic currents, wind patterns and the local geography and ecology plays a major role. Also, the release of methane from the lake seems to be an important part. I would also bet that being near the greatest rainforest in the world, not so far away nor too close to a large mountain range and also near a isthmus that connects two large continental masses separating two large oceans that almost but do not touch there to be something important. However, current-day science still does not seems to know enough to determine that with any certainty.
If I could handwave the geography of such a place, I would create a water cycle that bottlenecks in/over your island, where wind patterns make hot and cold air currents meet. There should be some way of placing oceans, continental masses, forests, rivers, lakes, mountain ranges, valleys, swamps and whatever else that will eventually produce the perpetual storm. However, I don't know what would be that way, and I think that nobody really does.