In XMPP, clients can register for a new account, using the XMPP connection itself. This is called In-Band Registration and is defined in XEP-0077. To enable this feature, you have to call the AllowRegistration() method, before connecting to the network:
You must also make sure to create the XmppClient using a proper password, not a password hash:
this.xmppClient = new XmppClient(Host, Port, UserName, Password, "en", typeof(App).GetTypeInfo().Assembly)
Once the server gets to the part of authenticating the client and learns that such an account does not exist, it will attempt to register the account, using the credentials provided. If successful, the proper password hash and ...