massaction on predefined fields

Hi,

In your action, you want to use the "string" mode, not the "operation" mode.
And in the input field on the right, you want to copy / paste the value to be used for all the products filtered.
Note that in a custom field, you have two columns for values, the "value" and the "title". You need to provide the "value" in the input field, not the "title".

Also, I would recommend adding an extra filter on the custom field column with the mode "=" and the input field empty.
That way, it will only fill the custom field of the products where that field is empty.