How to assign a value to a preset variable in IBM Watson Assistant

I want that when I start my system and the Watson chatbot opens, my user_name variable already has the name of the user who has logged in.

onLoad: async (instance) => {
 
       /*Here I get the user name from HTML*/
        let userName = document.getElementById("user_name").value || '';
        instance.restartConversation();
        instance.updateHomeScreenConfig({
            is_on: true,
            greeting: `Hola ${userName}! Soy tu asistente virtual, dime algo y te ayudo!`
        });
        instance.send({
            input: {
                text: ""
            },
            context: {
                global: {
                    system: {
                        /*Here is my problem*/
                        user_name: userName
                    }
                }
            }
        }).then((eve) => {
            console.log("Nombre del usuario enviado al contexto:", userName);
        }).catch(err => {
            console.error("Error al enviar el nombre al contexto:", err);
        });
 

        await instance.render();
    }

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If it shows me the greeting correctly, but I want my variable to update its value, I already created the variable in Watson, and I also have an intention that tells me what my name is.

That my variable is initialized correctly