How do you draw freehand (like a pencil tool) in LibreOffice Draw?

There is a tool bar down the left side. It includes a "pencil and line" symbol, highlighted in the image below:

The default setting is freehand line, but if you click on the pull-down arrow, you can select other types of lines.

Click on that tool icon. When the cursor is over the workspace, it will change to a crosshair. Position it where you want to start, then click and drag where you want the line. Release the button when you're done.

BTW, if you mess up, you can reselect the line (if it isn't still defined by an object box), and delete it. So you can practice and explore the features and options and then delete it without affecting anything.