The Button id specifies the button to be used for the 'flow collapse' button inside a Panel. Button ids can be selected from the dropdown next to the property. In order to see a button in the dropdown, a button control must first be defined inside a PanelHeader or PanelFooter container inside the Panel.
For example, here is a defined [Button] control from the 'Other Controls' menu. It has been placed inside a PanelHeader inside a Panel.
The [Button] control has an Id: 'MY_FLOW_COLLAPSE_BUTTON'.
When the [Panel Card] containing the header and button is highlighted, and "Has a 'flow collapse' button" property has been checked, then the 'MY_FLOW_COLLAPSE_BUTTON' will appear in the dropdown. It can then be assigned as the 'flow collapse' button.