Maybe You can do like this?

# Maybe there is a typo in your module name :slight_smile: 

defmodule AppName.on.Worker do
    use GenServer

    # THIS IS API

    def start_link do
        # receives the message as argument
        GenServer.start_link/3
    end

    def some_task, do: GenServer.call ...

    # THIS IS SERVER SIDE

    def init/1 do
        send self(), {:check_args, message_from_mqtt_as_map}
        {:ok, state}
    end

    # Wherever You need it...
    def handle_call(:some_task, _from, _state) do
      perform_some_task()
    end

    def handle_info({:check_args, message_from_mqtt_as_map}, state) do
        case message_from_mqtt_as_map["action"] do
            :create ->
                do_create(message_from_mqtt_as_map)
                perform_some_task()
           ...
        end
        {:noreply, state}
    end

    # helper functions for the task
    defp perform_some_task, do: ...
end

BTW Why do You need to do this

send self(), :some_task

instead of a simple function call?