getMostStarredRep.js

function getGitHubRepoUrl() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var range = sheet.getActiveRange();
  var username = range.getValue();
  var adjacentCell = range.offset(0, 1); // adjust this if the URL is in a different column
  var repoUrl = "";

  if (username.indexOf("github.com") >= 0) {
    username = username.replace(/^.*com[/]([^/]*).*$/,'$1');
  }

  var apiUrl = "https://api.github.com/users/" + username + "/repos?per_page=1000";

  var response = UrlFetchApp.fetch(apiUrl);
  var data = JSON.parse(response.getContentText());

  var maxStars = 0;

  data.forEach(function(repo) {
    if (repo.stargazers_count > maxStars) {
      maxStars = repo.stargazers_count;
      repoUrl = repo.html_url;
    }
  });

  adjacentCell.setValue(repoUrl);
}