How to create a pre-populated custom text field

THIS IS A CUSTOMPRESS REQUEST - BUT VIA WPMUDEV DASHBOARD IT CONNOT BE SELECTED BECAUSE IT WAS INSTALLED ALONG WITH DIRECTORY

Hi,

I am trying to create a new custom text field that contains text in the field so users can edit or add to it on the post edit form. (not placeholder text that will disappear.)

Can you please advise.

Many thanks
Chris