820TiP ENGOT-EN6/GOG-3031/NSGO-CTU-RUBY part 2: A phase III, randomized, double-blind, study of dostarlimab + carboplatin-paclitaxel followed by dostarlimab + niraparib versus placebo (PBO) + carboplatin-paclitaxel followed by PBO in recurrent or advanced endometrial cancer (EC)