I have never tried the linked program as it's windows only......
I tested a few things just using the built in capacities in OpenCPN, and my conclusion is, that it's faster to do a position line manually than using a script.
1. Use JesperWe's instruction for the first wpt. This is your assumed position, as used in calculating your azimuth and intercept.
2 Start the "Route" tool, an use the created wpt as a start.
3 Draw the intercept line along the azimuth direction. Drop waypoint No2.
4. Continue the next leg at a right angle to the first. Drop wpt No3.
5. Double back through wpt2 and past. Drop wpt 4.
6. The leg wpt 3 to wpt 4 is your position line.