Loading the plugin with packadd

Hello,
I normally manage Vim plugins as Git submodules and then load them using `packadd`, like this:

```
packadd! terminus
packadd! corpus
packadd! vim-zsh
```

What I'm trying with `markdown-preview` is:

`packadd! markdown-preview, { 'do': { -> mkdp#util#install() } }`

Any way I can make it work this way?

Thank you!