External MQTT Broker Setup

i am trying to connect lora server to application via MQTT, i am following the rules in this link:
but i have a problem that i don’t know how to access the server in this image

can any one help me ?
(p.s i am using Rak2245 pi hat, and i tried to connect through the address it only connect to chirpstack which doesn’t has the settings in the document to follow on it)

As mentioned in the article, you only need to install MQTT client to access it, but you can’t understand why you need to connect to it by address?

This interface is from RAK Industrial Gateways (RAK7249, RAK7258 etc.) . On Pi with RAK2245 you anly have ChirpStack server or you can use TTN as Server.
Can you explain what you try to achieve, so we can help better?

i want to send try to send data from the server to the node by MQTT

You need only to send message to MQTT broker that communicates with the server and gw bridge.
You can do this by log in to the RAK2245 pi and use commands mosquitto_sub and pub. For Gateway topic use
mosquitto_sub -t gateway/{{eui}}/rx -v (change the eui with Gateway EUI)
But this is only valid when you register the Node in ChirpStack App Server.
So lets start from here https://doc.rakwireless.com/rak2245-pi-hat-edition---lorawan----gateway-concentrator-module/accessing-your-gateway

Feel free to ask if you have some questions.