The CB Gallery auto action will let you create a new media entry, but it will only create link entries. It does not support uploads. You would have to code such a use case yourself using a Code action to deal with uploading the file.

You wouldn't use a trigger for this. You'd need to authenticate them like you would for any API endpoint and how you do that is up to you. Then you'd call the auto action URL directly. Since you'd be using a token field you could set User parameter to Custom then in the custom user PHP find the user based off that token and return either their user object or user id and that user would then be used for the auto action.

If you send the file through with a proper file POST header you can just call CB Gallery API in a Code action and it'll handle the upload itself otherwise you can place the file in the appropriate folder that CB Gallery expects it to be in (see /images/comprofiler/cbgallery) then create a database entry with the relevant information about the file (either directly or by using CB Gallery API). Linking would not be the appropriate method for uploads.

You'll need to work with gallery item table object. Specifically the below class.

/components/com_comprofiler/plugin/user/plug_cbgallery/library/Table/ItemTable.php

Example of how to initialize such object.

Review the class provided above for further usage information. It can process the upload automatically in ->store if you set the input data into _input and the files data into _files. Example as follows.

That would work with a standard HTML form. So you'll need to adjust it however necessary based off your usage. _files is specifically the PHP global $_FILES.