While the hopper is intended to be used for automation it is possible to manually store items within it by pressing the 'Store' key (default 'T'). The selected item or stack in the hotbar will be stored in the hopper if there is space. You can also store 10 items only by pressing Shift+T. To store an exact amount you can split the stack in your inventory (hold Shift and drag). Each press of T will insert the last matching stack in your inventory.