Auto-completion HOWTO :

For making work the auto-completion (type Ctrl-Space to trigger the autocomplete list box, see screenshot), user has to add his own api file in the directory YOUR_NPP_DIR\plugins\APIs where YOUR_NPP_DIR is the directory in which you installed Notepad++. Note that the extension of api file is api. Here is the list of api file names that they should be for all the supported languages :

As you can see, for 3 languages HTML, SQL and RC, you don't need the api file, they're already hard coded. An api file is a simple normal text file, so you can create it with whichever editor (of course your favorite one - Notepad++ :-) ). Note that you should create your api list in alphabetic order. You can find an example php.api in the directory YOUR_NPP_DIR\plugins\APIs.

From the v2.7, the Auto-Completion for the User Defined Language is supported. Just name the api file as your defined language name.

ex : if your defined language name is "My Lang", you have to create your api file named "My Lang.api" in order to perform Auto-Completion operation.