Something not right here with Kate.

app/controllers/credentials_controller.rb

@@ -1,5 +1,6 @@
 class CredentialsController < ApplicationController
   before_action :set_credential, only: %i[ show edit update destroy ]
+  before_action :set_domain, only: %i[ new ]
 
   # GET /credentials or /credentials.json
   def index
@@ -63,6 +64,10 @@ class CredentialsController < ApplicationController
       @credential = Credential.find(params[:id])
     end
 
+    def set_domain
+      @domain = Domain.find(params[:domain_id])
+    end
+
     # Only allow a list of trusted parameters through.
     def credential_params
       params.require(:credential).permit(:email, :password)