How to set value in newly created entity form (using Utility.openEntityForm)

Hi,

I'm hoping someone can help, as I just can't figure this out.

I'm using Dynamics 365; and I have created a button in the ribbon that allows users to quickly create a new Phonecall record without having to navigate much.
I'm using this code, which I found online:

Xrm.Utility.openEntityForm("phonecall", null, {_CreateFromId: Xrm.Page.data.entity.getId(), _CreateFromType: Xrm.Page.context.getQueryStringParameters().etc}, {openInNewWindow: true});

It works and it sets the Regarding to the parent (incident), something I struggled with until I found that code.

However, there is another field on the Phonecall that I need populating, but I can't work out how to do it.
So, on this newly created (but not yet saved) Phonecall, there's a textfield called "new_contactname" and I want to populate it (with a value from the parent, which I know how to get), but how do I address this field on this newly opened record?  I've tried all sorts, but clearly not the correct sorts!

Thanks for any advice!!