The problem is that you are defining the buttonCity variable in the TWeather function so that variable is only available in the scope of that function. Please read the following: https://www.w3schools.com/js/js_scope.asp

You will have to assign this variable where it is accessible to the other function as well.

Example:

let currentCity = '';

TButton.addEventListener("click", TWeather);

function TWeather(){
    outputDiv.style.display = "block";
    currentCity = "Toronto";
};

And then API CALL:

const url = "https://api.openweathermap.org/data/2.5/weather?q=" + currentCity + "&appid=" + myAPIkey + "&units=metric";

I think that const is a local variable not a global one. Define your variable after the addEventListener outside the function and give it a try.

You did not specify what you wanted to do with the URL so I just did a console.log.

Here I create a object and add some properties including some functions to add the event handler. I also added the target city to each button as a data attribute, then pulled that out within the event handler.

This is a bit overkill perhaps but does illustrate some ways to avoid global variables and make it so you can add more buttons to the HTML without any code changes.

var myapp = {
  url: {
    base: "https://api.openweathermap.org/data/2.5/weather?q=",
    api: "&appid=",
    unit: "&units=metric"
  },
  apiKey = "key",
  baseSelector:".weather-toggle",
  buttons: {},
  init: function() {
    const app = this;
    this.buttons = document.querySelectorAll(this.baseSelector);
    this.buttons.forEach(function(button, index, buttons) {
      button.addEventListener("click", this.toggleWeather, false);
    },this);
  },
  toggleWeather: function(event) {
    const targetCity = event.target.dataset.targetCity;
    const url = myapp.url.base + targetCity + myapp.url.api + myapp.apiKey + myapp.url.unit;
    console.log(url);
  }
};

myapp.init.apply(myapp);

<button class="weather-toggle" type="button" data-target-city="Toronto">Toronto</button>
<button class="weather-toggle" type="button" data-target-city="NewYork">New York City</button>
<button class="weather-toggle" type="button" data-target-city="Boston">Boston</button>