To which workspace? Each function has its own one. Do you mean the base-workspace, which is available in the command window? Then:
assignin('base', 'A', 123 + 17)
Please note, that using such remote-creation of variables frequently will make it much harder to debug your program. Imagine you have tweo different GUIs which assign the same variable. Then it is impossible to find the function, which has applied the last changes.