When working with ProStructures elements it is always best to turn on the ProStructures snaps. This wiki explains how to enable the ProStructures snaps and it's usage.
1. Open the Global Settings or Options by holding the right mouse button in the View window and then click on the Options. Or search ‘Global Settings’ in the Search Ribbon and click on it to open it.
2. Left-click on the Snaps page and set the settings as shown in the below image. ProStructures Snaps, ProStructures Legacy Snaps and ProStructures Tooltip can also be enable or disable from the PS Toggles.
Now for the snaps you have following three options.
ProStructures snaps, which will magnet to the points shown here switching between them using the keyboard arrows. They will magnet to the start and end points and any of the closet points along the length of the object according to the Keypoint Snap setting (enabled with the “K” key).