Welcome 1940LaSalle,
in LibreCAD you can find the polyline join tool in the polyline menu or toolbar.
Where the menu or toolbar is located depends on the version. Things were reordered with version 2.1.
But the tool is named "Create Polyline from Existing Segments" in all versions, so you may find it.
For the current version you can use the online manual for more information:
https://librecad.readthedocs.io/en/latest/ref/tools.html#polylineSadly, it seems, that there is no command for the command line for it.
Armin