Connections only work if you are already in a fiber. This is the case in a connectionHandler, but not from your main function. You'll have to make your own worker fiber. (But tbh if you only have one connection anyway, you might as well use a standard Socket.)

If you are already in a connection handler set in the listen family of functions, you're all set - those are automatically in fibers. If you are in main though, you need to make a worker fiber.

Making a worker fiber is simple enough. You can do it with new Fiber or with FiberManager.makeFiber (the latter just calls the former with a size argument set up in the FiberManager constructor).

auto fm = new FiberManager();
fm.makeFiber(() {
	auto socket = fm.connectTcp4(...);

	socket.send(...);
}).call(); // you must call it the first time yourself so it self-registers

OR

import core.thread.fiber;

auto fiber = new Fiber(() {
	auto socket = fm.connectTcp4(...);
	// do stuff in here
}).call(); // same deal, still need to call it the first time yourself to give it a chance to self-register