Pete,

There are two issues ... audio and control.

For audio, one generally needs to route audio from the SDR to
WSJTX using one of the "Virtual Audio Cable" products. Your
SDR software should have an audio output selector which would
be set to the input of a VAC while the "Soundcard Input" of
WSJTX would be set to the output of that VAC.

For control, your SDR software will need to provide a CAT interface
that emulates a "well known" transceiver - generally the TS2000 -
and provides the frequency of your "FT8 slice" as *VFO A*. Note:
if you plan to leave the receiver on one frequency/band and not
band hop, you can avoid the CAT issues (WSJTX -> Radio -> Rig:
None).

73,

... Joe, W4TV