I haven't tried this exact setup, but I think the following should work. Connect the - control terminal of the SSR to the PS_ON output (the middle pin of the external 5V input connector) and the + control terminal of the SSR to either +5V (pin 1 on the expansion connector), +3.3V, or +12V. Most SSRs accept anything between 3V and around 30V on the control terminal, so it shouldn't matter which. Connect the AC terminals of the SSR in series with the live side of the AC mains feed to the PSU. Also connect a heavy duty mains-voltage momentary-action push button in parallel with the SSR AC terminals. I suggest at least a 250V 10A AC rating for the push button because of the current surge taken by the power supply when it starts up.
Put command M80 right at the start of the config.g file.
To turn the printer on, press and hold the push button for about 1 second. To turn it off, execute M81.
HTH David
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].