The documentation describing the use of Eikon COM APIs in Microsoft Office is available under the namesake section on this portal.
The Developer Guide is available under the Documentation tab and a set of tutorials is available under Tutorials. There's a specific tutorial on contributions. Programmatic contributions using Eikon COM APIs are only available through TREP infrastructure. Direct contributions over the Internet, which are possible using =RtContribute function in Excel, are currently not available through any Eikon APIs.