You connect a cable that provides a USB Type A Female connector to one (or both) of the internal USB 2.0 headers on the BE NUC's board.
The locations of the two USB 2.0 headers is detailed in Figure 2 on Page 4 of the BE NUC's Technical Product Specification (TPS). The pin-out of these headers is detailed in Section 188.8.131.52 on Page 39 of this TPS.
I believe that such a compatible cable, produced by GORITE, is available from MicroSATACables. Here is link to the web page for this cable on their site: MicroSATACables.com: 3-Inch Internal NUC USB 2.0 Cable with USB A Female to 4-Pin Connector. MicroSATACables and GORITE are also vendors on Amazon and you can purchase this cable there as well: Amazon.com: 3-Inch Internal NUC USB 2.0 Cable with USB A Female to 4 Pin Connector.
As I said, I believe that this cable is compatible. Unfortunately, I cannot find pin-out information on the MicroSATACables or GORITE sites to verify for-sure. I suggest that you ask MicroSATACables and/or GORITE for verification before purchasing.
Hope this helps,