In the Fill layer setting for LightBurn, change the Ramp Length value to something other than zero. That’s it. My interface at the moment is going to look a lot different than yours as we’ve reorganized a bunch of things for the next release, but that’s really all there is to it.
Once you set a ramp length there, do a preview, and enable the ‘Shade according to power’ switch to see the effect:
The fuzzy fade around it is the ramp. You will also need to make sure that your min power is set lower than max power for it to work - the ramp is a fade from min to max, so if they’re the same, you won’t get a ramp, just “wider” output.