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.
Editing the file in Advanced Mode, insert this code right below this line:
<asp:Content ContentPlaceHolderId="PlaceHolderMain" runat="server">
Next, the script.
And this is it. Thanks to SPUtility, this works for text fields, dropdown menus and more.
Autopopulate a SharePoint Form from URL (with SPUtility.js)