At the moment, it looks like you have two options. For both of these you first need to define a link in your URDF at which you want the marker to be placed, e.g. your_robot_tool_point_link.

A) Without end effector group

Define your robot planning group using a chain of links like this in the SRDF, and have no end effector for this group:

<group name="your_robot">
    <joint name="your_robot_joint_1" />
    <joint name="your_robot_joint_2" />
    <joint name="your_robot_joint_3" />
    <joint name="your_robot_joint_4" />
    <joint name="your_robot_joint_5" />
    <joint name="your_robot_joint_6" />
    <chain base_link="your_robot_base_link" tip_link="your_robot_tool_point_link" />
</group>

The marker will be placed at the tip_link frame if no end effector has this group as its parent, or has one of the links set as its parent_link.

2) With end effector group

If you define an end effector group in your SRDF, the interactive marker will be placed at the parent_link, so you can write this:

 <end_effector name="your_robot_ee_name" parent_link="your_robot_tool_point_link" group="your_robot_ee_group" parent_group="your_robot" />

If you want multiple interactive markers on your robot, you can define multiple end_effector tags that use the same your_robot_ee_group. If you don't have a group, you can define this hacky one, although you can't use this to plan:

<group name="your_robot_dummy_group">
    <joint name="your_robot_joint_1" />
</group>

As Felix already pointed out, it is possible to have even multiple markers by defining multiple end_effectors:

<end_effector name="tool_mount" parent_link="tool_mount" group="hand" parent_group="arm"/>
<end_effector name="tool_tip" parent_link="tool_tip" group="hand" parent_group="arm"/>