You should follow these steps for inserting an element with a content reference
Start one of the actions Insert a DITA Content Reference and
Insert a DITA Content Key Reference.
In the dialog Insert Content Reference select the file with the
referenced content in the URL field.
In the tree that presents the DITA elements of the specified file that have an
id attribute you have to select the element or the interval of elements
that you want to reference. The conref field will be filled automatically
with the id value of the selected element. If you select an interval of
elements the conrefend field will be filled with the id
value of the element that ends the selected interval.
Press the OK button to insert in the current DITA file an
element with the same name and with the same conref attribute value (and
optionally with the same conrefend attribute value) as the element(s)
selected in the dialog.