As Jeremy Crawford said you can use interaction with object when you store a spell in a mundane item.

Does that also mean I can use my free action to activate it?

No.

The Use an Object action is not a Free Action. They do similar things. The Free Action are certain things you can do as part of another action or movement on your turn.

"You normally interact with an object while doing something else, such as when you draw a sword as part of an attack. When an object requires your action for its use, you take the Use an Object action. This action is also useful when you want to interact with more than one object on your turn."

Blue is the free action. Red is the Use an Object. The difference is that the object or feature specifically states you must use an action.

From the Spell-Storing Item feature:

"While holding the object, a creature can take an action to produce the spell’s effect from it, "

Sorry, but no casting spells as a free action.

Thanks for this professional answer!

I couldn't really differentiate between Use an Object and interaction with object before.