There are two ways to hand over an image from Lightroom Classic to PhotoLab. They depend on how DPL is installed and how it is called.
Define DPL as an external editor in LrC’s settings.
Send image(s) to DPL, e.g. with the context menu. Note that RAW images will be converted first, before being sent to DPL. Converted images can contain Lightroom edits.
Make sure that you can see the DPL plug-ins in Lightroom’s module manager. DPL should ask to install the plug-ins on first launch. If you have declined to install the plug-ins, you can export an image to LrC with “export to application” and DPL should ask to install the plug-ins again.
Send image(s) to DPL using the module option located in DPL’s file menu. Note that DPL will edit the RAW file without any Lr edits. After editing in DPL, export the images in JPEG, TIFF or preferably DNG format.
Edit the image(s) in DPL: Make DPL the active window and enjoy the works!