Add prefilled custom field

Hi,

I want to have a custom field that is partial pre-filled.
The Key should be given and the value should be insert by the user.

I’m creating a plugin that let’s users insert a gallery.
After the gallery is created the user should insert the gallery id to a custom field. In the template the custom field inserts a value to a shortcode.

So what I’d like to have is:

$gallery_id => $value

But I can’t figure out how to create a custom field where the $key is allready filled.