I use the <Export Route> option in Route
Manager in OpenCPN to export the routes and waypoints as a gpx-file. I open this gpx-file in GPS Utility so I can upload to the GPS.
Using the <Send to GPS> option in Route
Manager doesn't work for me cause my Furuno
GP33 needs some specific propriatery codes.
IIRC Dave wrote something about adding the various formats GPS's use to OpenCPN in the future but imo he is spending his time on more valuable subjects and this GPSU solution works great after the author of GPSU added the Furuno
GP33 to the supported types list.
I have the registered version of GPSU which cost me a $50 or so some 10 years ago. In the GPS-Setup I see Raymarine mentioned. So you could download the freeware version and try it.
If it doesn't work with your plotter I am not sure if the author will add a specific brand/type of device when you use the freeware version of his program but you can ask him. www.gpsu.co.uk