In order to access leader key bindings from EXWM apps you can do something like this:

(push ?\M-m exwm-input-prefix-keys)

However, it’s probably would be more convenient to set dotspacemacs-emacs-leader-key to be “s-SPC” and then instead of the snippet above, do this:

;; note double whitespace after the second backslash
(push ?\s-\  exwm-input-prefix-keys) 

then pressing Cmd+Space would be the equivalent of pressing SPC in “normal” mode

in order for “simulation keys” feature to work properly you may need to do the following:

(evil-set-initial-state 'exwm-mode 'emacs)

Spacemacs layouts (AKA persps) can mess things up for floating windows, you may need this:

(setq persp-init-frame-behaviour nil)