Access external REST API

Tristan gave already a pretty good answer. I wanted to add my guide Data integration options with Cumulocity IoT which might give you some starting point understanding the basic concepts & options you have to integrate data.

Especially the Service Integration seems to be the one you are looking for.

If you are not the technical guy and don’t want to develop anything but do this on configuration level you might check out one of the 3rd party tools mentioned here