Spawn process and send data to STDIN when data avilable

I want to spawn a process (golang binary), capture its STDOUT and have a reference to STDIN which i can send to at any time. In my case i am getting data in via Websockets and want to pass them to STDIN of the spawned process.

I am having issues reusing STDIN with various methods i tried, like using PTY.spawn

input = nil

def run
  cmd = "app/bins/transcode" 
  read, write, pid = PTY.spawn(cmd)
  input = write
  Signal.trap(:WINCH) { write.winsize = STDOUT.winsize }

  read.each do |line|
    # THis works fine and sends the spawned process' STDOUT over websockets
    ActionCable.server.broadcast 'shell_channel', test: line
  end
  Process.wait(pid)
end

# This is never sent to STDIN of the spawned process
input.puts "hello"

I tried additionally to use a lock syntax with PTY and popen3 but both failed to receive STDIN.