gtk-entry-completion-insert-action-text (completion index text)

completion -- a gtk-entry-completion object
index -- an integer with the index of the item to insert
text -- a string with the text of the item to insert

Inserts an action in the entry completion's action item list at position index with the given text. If you want the action item to have markup, use the function gtk-entry-completion-insert-action-markup.