How to fix end effector direction to approach object with MoveIt

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Hello, My question is that how to fix end effector orientation to approach object(move forward) with MoveIt?

I already let my robot to move to a given goal position(x, y, z). If I would like to let robot grasp the object, move and approach it slowly is necessary. So I want to move the robot 15cm forward by fixed direction of end effector.

As shown in the picture:

Does MoveIt or others have any function can achieve this? If you can provide me example code with python will be better.

Thanks a lot.