Check A Check-Box on 3rd Party Site Chrome Extension

I'm trying to make a Google Chrome Extension that will automatically check the 3rd checkbox (Hansen Dam) on a 3rd party website. I can't seem to get it to work.

Here's the website: https://cityofla.ezlinksgolf.com/index.html#/preSearch

manifest.json

    {
  "manifest_version": 2,
"name": "Tester",
"version": "1.0.0",
"permissions": ["activeTab"],
  
"content_scripts": [
  {
    "matches": ["<all_urls>"],
    "all_frames": true,
    "js": ["Scripts/content.js"]
  }
]
}

content.js

// trying to check the 3rd box directly, method 1
const a=document.querySelectorAll('input[type="checkbox"]');
a[3].checked = true;



// i've tried checking all the boxes with a loop, method 2
var arr = [];
const allInputs = document.getElementsByTagName("input");
for (let i = 0; i < allInputs.length; i++){
    if (allInputs[i].type == 'checkbox')
      allInputs[i].checked = true;
      arr.push(i);  // make a new array of only checkboxes, to pick 3rd checkbox from this array
}

// trying another way, method 3
arr[3].checked = true;

Any help appreciated!