This is a script for using in the Draft app on macOS. It toggles the checkbox state on selected lines.

script.js

// This is a script for using in the Draft app on macOS
// It toggles the checkbox state on selected lines

// Toggle tasks marks on selected lines
var offState = "- [ ]";
var onState = "- [x]";

// Normalize leading spaces to multiples of 4 (0, 4, 8, 12, ...)
function normalizeIndent(spacesCount) {
  return Math.floor(spacesCount / 4) * 4;
}

// grab state
var [lnStart, lnLen] = editor.getSelectedLineRange();
var lnText = editor.getTextInRange(lnStart, lnLen);
var [selStart, selLen] = editor.getSelectedRange();

// just add mark if empty line
if (lnText.length == 0 || lnText == "\n") { 
  editor.setSelectedText(`${offState} `);
  editor.setSelectedRange(selStart + offState.length + 1, 0);
}
else {
  // create line array and tracking vars
  var lines = lnText.split('\n');
  var startOffset = 0;
  var lengthAdjust = 0;
  var flTrailing = false;
  if (lines[lines.length - 1] == "") { 
    lines.pop();
    flTrailing = true;
  }
  var newLines = [];
  const re = /^(\s*[-\*] )?(\[ \]|\[x\])?(.*)/;
  const containsRe = /^(\s*[-\*] )?(\[ \]|\[x\])(.*)/;

  // determine if we are removing or adding marks
  var flRemoving = true;
  for (var line of lines) {
   if (line.length > 0 && !line.match(containsRe)) {
     flRemoving = false;
    }
  }

  if (!flRemoving) {
    // add marks
    var isFirst = true;
    for (var line of lines) {
      const match = re.exec(line);
		if (match[2] || line.length == 0) {
      	  newLines.push(line);
      }
      else {
        var prefix = match[1];
        var suffix = match[3];
        if (!prefix) { prefix = ""; }
        if (!suffix) { suffix = ""; }
        
        if (prefix) {
          // Already has a list marker (- or *), just add the checkbox
          // Normalize leading spaces to multiples of 4
          var prefixLeadingSpaces = prefix.match(/^(\s*)/)[1];
          var prefixContent = prefix.trimStart(); // "- " or "* "
          var normalizedSpaces = normalizeIndent(prefixLeadingSpaces.length);
          var spacesRemoved = prefixLeadingSpaces.length - normalizedSpaces;
          var normalizedPrefix = " ".repeat(normalizedSpaces) + prefixContent;
          newLines.push(`${normalizedPrefix}[ ] ${suffix}`);
          if (isFirst) {
            startOffset = 4 - spacesRemoved; // "[ ] ".length minus removed spaces
          }
          else {
            lengthAdjust += (4 - spacesRemoved);
          }
        } else {
          // No list marker, add full offState (- [ ])
          // Normalize leading spaces to multiples of 4
          var leadingSpaces = suffix.match(/^(\s*)/)[1];
          var suffixContent = suffix.trimStart();
          var normalizedSpaces = normalizeIndent(leadingSpaces.length);
          var spacesRemoved = leadingSpaces.length - normalizedSpaces;
          var normalizedIndent = " ".repeat(normalizedSpaces);
          newLines.push(`${normalizedIndent}${offState} ${suffixContent}`);
          if (isFirst) {
            startOffset = offState.length + 1 - spacesRemoved;
          }
          else {
            lengthAdjust += (offState.length + 1 - spacesRemoved);
          }
        }
      }
      isFirst = false;
    }
  } else {
    // remove marks
    var isFirst = true;
    for (var line of lines) {
      if (line.trim() == "") {
        newLines.push(line);
        continue;
      }
      const match = re.exec(line);
      var prefix = match[1];
      var suffix = match[3];
      var state = match[2];
      if (!prefix) { prefix = ""; }
      if (!suffix) { suffix = ""; }
      if (suffix.startsWith(" ")) { 
        suffix = suffix.substr(1);
        if (isFirst) { startOffset -= 1; }
        else { lengthAdjust -= 1; }
      }
      newLines.push(`${prefix}${suffix}`);
      if (isFirst) {
        startOffset -= state.length;
      }      
      else {
      	  lengthAdjust -= state.length;
      }
      isFirst = false;
    }
  }

  // update text and selection
  if (flTrailing) {
    newLines.push("");
  }
  var newText = newLines.join("\n");
  editor.setTextInRange(lnStart, lnLen, newText);
  editor.setSelectedRange(selStart + startOffset, selLen + lengthAdjust);
}