Pickup animations can be set in the ItemList, select the item row you want to add an animation to and in the details panel find the Pickup Animation.

You animation must be a montage and in the montage you must have the slot set to UpperBodyPostAO this will mean the animation will play on the upper body of the character, this allows the characters legs to still move when then player moves while the animation is playing.

If you want the animation to play on the full body of the character you can use the DefaultSlot keep in mind that this means if the player moves there legs will not animate until the pickup animation is finished.

You can set the montage slot in the montage animation copy the video below.