You can already find this process on other sites around the web, but it was only in a comment I found out that this only works with default forms (and not with custom made forms created by you). This is why you need to edit NewForm.aspx, DispForm.aspx or EditForm.aspx.

Also, to be able to use this you must be a Site Collection administrator.

This is done using prototype.js and SPUtility.

Click here to see the code.