gistfile1.js

// Drafts 4 script step to create custom tags
// using the a line or a default value if that line is empty or missing

// split draft into an array of lines
var lines = draft.content.split('\n');

// reusable function to define tag based on line or default string
function createLineTag(lineNumber,default) {
  var tagName = "line"+(lineNumber+1).toString();
  if(lines[lineNumber] && lines[lineNumber].length > 0) {
    draft.defineTag(tagName, lines[lineNumber]);
  }
  else {
    draft.defineTag(tagName, default);
  }
}

// call for each line you want a tag for...
createLineTag(0, "first line default string");
createLineTag(1, "second line default string");

// templates in steps after this script step in an action
// can use [[line1]], [[line2]] tags to get line or default values.