How to get custom button in the Actions?

I am trying to get the custom button in the actions of purchase order as shown in the below image.

I was able to get this button by changing core code(purchase_order_list.js).
I tried to add the purchase_order_list.js inside my custom app(custom/purchase_order/).But,this is not working for me.

is there any way to achieve this,i.e get a button inside actions of purchase order.