An association is defined as a data attribute of the HTML tag. Instead of passing the reference to another control you define the ID of another control.
#!html <div data-sap-ui-type="sap.ui.commons.Label" data-text="Message:" data-label-for="message"></div> <div data-sap-ui-type="sap.ui.commons.TextField" id="message"></div>
The code snippet defines the link between Label and TextField by using the ID of TextField as a value for the data-label-for attribute of the Label.