Hi Szymon,
This is actually not an API but a feature of Bing.com/maps. You posted in the correct place and I apologize that someone redirected you here. I will give you the same answer I have provided you there. "sp" is designed for a points collection. Your point will work if you use the setting "cp" rather than the "sp". See my answer at your original post: https://answers.microsoft.com/en-us/bing/forum/all/custom-maps-url-doesnt-work-for-random-coordinates/df224c97-b06d-4e72-9236-0850b2d4d011
Try:
https://www.bing.com/maps?cp=51.236987%7E22.671661&lvl=11.0
Sincerely,
IoTGirl