You need to create a variable from the Variable manager. It can be a list or File variable type. What matter is the change policy. In your case if you would like your virtual user to use a value and keep it during all the iterations then you shoud select "For each virtual user" as change policy.

But if you would like the user to use a different value on each iteration then you need to choose "on each iteration".

Remember that the user will loop the Actions container. Each loop is an iteration.