Every element has some properties. These properties are all read-only.
To insert an element property into expression, you can click the button beside the input field and choose "Insert Element Property..." in popup menu.
You will be asked to choose an element to provide the property. When the window below shows up, you can click on the editing area to choose element(s). You can also type element id in the input field.
After choosing the element, you can select the property from a drop-down list: