hi there!
to answer your question, you can actually use an expression instead of just entering text; to get the value of a private variable from an object, you could use e.g. Sprite('variableName')
but i would really like to recommend that you try some of the tutorials (link), which cover this and similar basic topics; they are immensely helpful in getting to know construct & the way it thinks, and usually don't steal too much of your time in the process.