Get value from selected option and pass it to the function

Hi im making a reservation page in razor pages asp.net. On my page I have two drop downlist. One for the location other for the rooms of that specific location. I want to pass the value of that selected location to the function which shows the rooms (function is named showRoom()), so that the rooms will be shown depending on which location you selected. How do i do that with html? I tried something but didnt really work. Here is the html code

Most efficient way to save a file on disk while executing async work, on a ASP.Net CORE Web Request

On a request on my Web API, I’m saving a image to the disk and also processing it with an external API which usually takes several seconds. This is a high traffic API, thus I would like to design it in the most efficient way. The image comes in Base64 “encoding”, but this is not pertinent. We can think about it as an arbitrary byte array of 150KB in average (so the saving to disk operation should be really fast).