I found no documentation on how to do this but found this option in the home-manager source code that I might not be using correctly:

home-manager.users.my_username.xfconf = {
  enable = true;
  settings."xfce4-keyboard-shortcuts" = {
    "<Super>space" = "rofi -show drun";
    "<Super>space" = "rofi -show drun";
  };
};

Any ideas?

Here is my full configuration.nix file for full context if that helps. I just started with Nix and NixOS this week so I the config is a bit haphazard at the moment.