I’m a new user of LibreOffice.
I should find out what the LibreOffice Writer API is like. Where can I find the documentation for the interface. Above all, I need information about Mail Merge, how can I control Writer with my program (Visual Foxpro).
Start from MailMerge service and MailServiceProvider service. See also How to send document via email from libreoffice, which has code that does not use mail merge, but discusses sending an individual documents (using different machinery).
This is no direct answer to your question, but maybe you don’t need the API.
I have a template-document (a writer file with field-names, to be replaced with data from a datasource). This is connected to a registered datasource of LibreOffice. This can be a csv or a dbase-file (and many other types). If your Foxpro-Software writes to this datasource, all you need to do is opening the writer-file and select “print”. Some clicks later you have mail-merge running.
So check, if you really need the api.