fix: handle default org id (#3769)

internal/api/ui/login/static/templates/register.html

@@ -42,7 +42,7 @@
             <label class="lgn-label" for="username">{{t "RegistrationUser.UsernameLabel"}}</label>
             <div class="lgn-suffix-wrapper">
                 <input class="lgn-input lgn-suffix-input" type="text" id="username" name="username" autocomplete="email" value="{{ .Email }}" required>
-                {{if .DisplayLoginNameSuffix}}
+                {{if .ShowUsername}}
                     <span id="default-login-suffix" lgnsuffix class="loginname-suffix">@{{.PrimaryDomain}}</span>
                 {{end}}
             </div>