How to specify a placeholder (ie. 'you@youremail.com') in edit form input field if it is empty

ken@kmplace.com

When the 'Create' or "Edit' form pops up, I'd like to display a placeholder in the blank fields.
Like using an HTML:

<input type="text" id="email" name="email" placeholder="you@youremail.com">

rf1234

}, {
    label: "yourFieldLabel",
    name:  "yourField", 
    attr: {
        class: "yourClass",
        placeholder: "whatever"
    }