Mostly you need to connect new beam to ends existing beams.

From menu Create
Sections
Straight. Click Ok. Click on
-Define Start/end explicitly.

In this case toolbar Positioning will be activated. You can select Element or just select left beam.

You will see word Start.

Click Ok. Click on
again. You will be prompted to select end of new beam.

Select right beam.

Click OK.
Credit to Said Babanli (Azerbaijan)