This is the standard-issue weapon for your profession. You have to equip it to be able to use it. Right-click the weapon to equip it. Click the Wear button on the menu to open your Wear Window. Look in the weapons section and confirm the weapon is equipped in your right hand.
Click the Action button on the menu to open your Action menu. Select the Attack Actions sub-menu to find available combat actions.
To put an action (e.g. Start Attack) on your shortcut bar, left-click-and-hold the action, and drag it to the wanted location on the bar. Do this with the Fling Shot action - your weapon's special attack.
To initiate combat, target an opponent , e.g. a Leet or a Reet of Paradise, and use the Start Attack button in your shortcut bar. Take care not to be too far away from your opponent. Use the special attack button to perform your weapon's special attack.