I am new to this and I would start from an example of the UR (in swing).
I would modify the interface of the example SimpleGripper, by adding a checkbox under the action section (as shown in the attached image).
no it is not possible to add a Check box beneath the Grip/Release Action of the GripperDriver.
You can add additional User Input using the available Capabilities (Force, Vacuum etc.) or add additional Input Options in the installation part of the GripperDriver-URCap.
You can find a GripperDriver Manual under the following directory, if you are using the StarterPackage: /home/ur/sdk/sdk-1.11.0/doc/gripper_driver.pdf