以下のような3つのAPIを作りたいとき

POST /api/v1/registration_requests
GET  /api/v1/registration_requests/:token
POST /api/v1/registration_requests/:token/accounts

`param` で指定する

routes.rb

namespace :api, format: :json do
  namespace :v1 do
    resources :registration_requests, only: %i[create show], param: :token do
      resources :accounts, module: :registration_requests, only: %i[create]
    end
  end
end

api_v1_registration_requests         POST /api/v1/registration_requests(.:format)                                      api/v1/registration_requests#create {:format=>/json/}
api_v1_registration_request          GET  /api/v1/registration_requests/:token(.:format)                               api/v1/registration_requests#show {:format=>/json/}
api_v1_registration_request_accounts POST /api/v1/registration_requests/:registration_request_token/accounts(.:format) api/v1/registration_requests/accounts#create {:format=>/json/}

となり、パスパラメータにプレフィックスがついて registration_request_token になってしまう

`path` で指定する

routes.rb

namespace :api, format: :json do
  namespace :v1 do
    resources :registration_requests, only: %i[create show], param: :token
    namespace :registration_requests, path: 'registration_requests/:token/' do
      resources :accounts, only: %i[create]
    end
  end
end

api_v1_registration_requests          POST /api/v1/registration_requests(.:format)                 api/v1/registration_requests#create {:format=>/json/}
api_v1_registration_request           GET  /api/v1/registration_requests/:token(.:format)          api/v1/registration_requests#show {:format=>/json/}
api_v1_registration_requests_accounts POST /api/v1/registration_requests/:token/accounts(.:format) api/v1/registration_requests/accounts#create {:format=>/json/}

となり、解決！