Because you want the button to be clicked once the page is loaded, you want to make sure the DOM element itself has actually loaded before executing your code.
You can read more here:
What is the DOM ready event?
The below code waits for the page to fully load before executing 'click' on the button.
Non-jQuery solution. It doesn't make sense to add jQuery to your page if you will only use it once.
window.onload = function () {
document.querySelector('.btn.btn-primary.btn-lg.btn-withdraw').click();
}
Alternatively, you can add an ID to the button as suggested by Sorikairo and subsequently trigger the click:
//Add ID to button
<button id="myButton" class="btn btn-primary btn-lg btn-withdraw">Text!</button>
//Click on load.
window.onload = function () {
document.getElementById('myButton').click()
}