These two units are really not designed to interface together like that because the GP1870F already has it's own fish finder built-in. The GP1870F is a NMEA2000 device and there is no compatible/accepted NMEA2000 PGN for the NMEA0183 TLL sentence from the FCV587. In other words, there's no way to plot waypoints on your GP1870F from your FCV587 using the 'mark' button.
If you really want to you can interface the two devices so that the FCV587 has position and can store up to 20 waypoints internally. To do that, the following items are needed:
1. IF-NMEA2K2 (NMEA2000 to NMEA0183 converter) to convert the NMEA2000 position to NMEA0183
2. FI-5002 (NMEA2000 junction box) to create a small NMEA2000 backbone; only needed if the boat doesn't already have a NMEA2000 backbone
3. 2ea of 001-105-790-10 (2 meter NMEA2000 female single-ended cable) to connect the GP1870F to the FI-5002 and FI-5002 to IF-NMEA2K2. There are longer lengths if necessary
Connect the yellow wire on the IF-NMEA2K2 to the yellow wire on the FCV587 power/data cable AND the green wire on the IF-NMEA2K2 to the green wire on the FCV587 power/data cable.
The other option is to move the FCV587's transducer to the GP1870F to use its fish finding capability. Then, when there's a location to "mark" on the fish finder, a waypoint can be dropped on the plotter screen.