I would use the airport identifier, if available, or the name on the sectional.
For example, on this section of NY sectional, you can see several (Pvt) airports, they are indicated by red R in a circle.
PASPORT is enterable in skyvector.com as a destination, as is WATERS. Entering them as a destination, skyvector will change them to an airport identifier, which you can then type in as a waypoint.
B&B, just north of PASPORT, can't be entered as anything I could find to start.
But, dragging the pink route line, it can be pulled over B&B and then it shows up with an identifier you can use. Same with the Unknown airport south of 9B1.
If you click on the Departure or Destination airport, skyvector will open more info for that airport, and from there a list of all airports and their identifiers in the state can be selected. B&B and Unknown are both listed there.