The value of the adminConnectionString option can be retrieved via the Azure portal:
You can select either the Primary or the Secondary connection string.
In production mode the admin connection string is used by the Security Backend, but as there is no Security Backend in development mode, the frontend accesses the Admin API on its own. For this reason the frontend requires the admin connection string.
Usage of Development Mode
Under no circumstances should you publish your web application in development mode. It allows anyone with access to the web application to perform any requests towards your IoT devices and it also exposes the admin connection string that must be kept secret.
Production Mode
For production mode you need to provide a security backend for managing access permissions. You also need a Grant Request Function that acts as an adapter between the ux4iot library and this backend.