How do I add a custom field in the frontmatter?

[Kingfisher12312]

I want to add a URL field in the frontmatter to hold the link to the page the note refers to.

    note_frontmatter_func = function(note)
      if note.title then
        note:add_alias(note.title)
      end

      local out = { id = note.id, time = os.date('%Y-%m-%d', os.time()), aliases = note.aliases, tags = note.tags, url = {} }

      if note.metadat ~= nil and not vim.tbl_isempty(note.metadat) then
        for k, v in pairs(note.metadat) do
          out[k] = v
        end
      end

      return out

This is how I wrote it. However, when I save the note, the field is automatically cleared.