The ExtJs SelectBox control is aliased by br.select-box, and can be used within templates as follows:

  <span data-bind="value:selectionFieldProperty, control:'br.select-box'"></span>

By default caplin.presenter.node.SelectionField#options are used to generate validation errors if the user types something other than one of the available options, but you can invoke caplin.presenter.node.SelectionField#allowInvalidSelections with true if you'd prefer options to act as a list of suggestions only.