can not add a name space to spawn node

edit

I have the following launch file:

<?xml version="1.0"?>

<launch>
    <!--<include file="$(find husky_gazebo)/launch/base.urdf.gazebo.launch"/>-->
     <param name="use_sim_time" value="true" />

    <!-- Send the Husky A200 Robot URDF/XACRO to param server -->
    <param name="robot_description" command="$(find xacro)/xacro.py '$(find husky_description)/urdf/base.urdf.xacro'" />

    <!-- Spawn robot in gazebo -->
    <node name="spawn_husky_model" pkg="gazebo_ros"  type="spawn_model" args="$(optenv ROBOT_INITIAL_POSE) -unpause -urdf -param robot_description -model mobile_base />

    <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher" ns="husky_ns">
        <param name="publish_frequency" type="double" value="50.0" />
    </node>




    <include file="$(find gazebo_ros)/launch/empty_world.launch"/>
    <node pkg="robot_pose_ekf" type="robot_pose_ekf" name="robot_pose_ekf" ns="husky_ns" >
      <rosparam>
        freq: 10.0
        sensor_timeout: 1.0
        publish_tf: true
        odom_used: true
        vo_used: true
        output_frame: odom
      </rosparam>
    </node>
</launch>

I want to add a name space to this node but I cant

 <!-- Spawn robot in gazebo -->
    <node name="spawn_husky_model" pkg="gazebo_ros"  type="spawn_model" args="$(optenv ROBOT_INITIAL_POSE) -unpause -urdf -param robot_description -model mobile_base ns="husky_ns" />

Why I cant add ? and how can I add a name space for this node ?