It’s getElementsByName. "Elements", plural, as you can have more than one element with the same name. It returns a NodeList, an array like collection of nodes. If you want to grab the first element with that name, you need to access the element at the zeroth index:

const button = document.getElementsByName('form: j_id15')[0]

button.addEventListener('click', (event) => {
  console.log('submit button clicked')
})

// you can use the click() method
button.click()
// or
document.getElementsByName('form: j_id15')[0].click()

<input type="submit" name="form: j_id15" value="Click me" class="btn btn-blue">

PS: If it’s in a form, you should probably subscribe to the form’s submit event, instead of button click.

Which function should I use in JavaScript to get the button with name = "form: j_id15"?

I already tried with getElementId, getElementName, but none of them worked. Does anyone have any idea how to do this?

The excerpt from the html page below:

<input 
  type = "submit" 
  name = "form: j_id15" 
  value = "" 
  onblur = "return validarCPFCNPJ (this);" 
  class = "btn btn-blue"
>