How can I send http request from not-main thread?

Can't really see what's wrong with your code without your code being posted, however:

Unity does not directly support threads. Everything must be called from the main thread. However, if you wish to use threads for certain calls, you will have to use a producer / consumer (or similar) algorithm for processing the data in a thread, but then store it in a queue for the main thread to process on each iteration through. You will need to be careful with this though to avoid race conditions and will need to implement locks and such on the queue (and associated variables you wish to send between threads).

If you let us know what it is you are trying to implement we will be able to assist you better with an implementation theory and code.