It is possible to map a key + mouse button combination to automatically close a shape while this is being created.
Steps to Accomplish
1. Open the "Workspace->Button Assignments" dialog
2. Select one key/button combination and replace the pre-assigned Key-In with "Close" in the "Action" field.
In the screenshot here below show the "close" key-in was associated to the "Shift+Reset" combination, which by default would be "Shift+Right Click", but other combinations are possible.
To close a SmartLine, which will then become a shape, the following key in can be used, instead of "close", in step 2:
reset;create shape manual;XY=#,#,#;XY=#,#,#;reset;place lstring
Currently two separate key-in are needed for Shapes and Smartlines and hence two separate key+mouse button combinations are required (e.g. "Shift+Reset" and "Alt+Reset") for using both short-cuts.