PowerPoint doesn't have custom keyboard shortcuts, though there are third-party add-ins that make it possible.
As a workaround, you can add the Insert Text Box command to the Quick Access Toolbar, where it automatically acquires a numeric keyboard shortcut. If the command is at the 5th spot from the left, Alt + 5 will run it.
- In PowerPoint, choose File>Options>Quick Access Toolbar.
- Change the Choose command from dropdown to Insert Tab.
- In the left-hand list, scroll down to Insert Text Box and select it.
- Click on the Add>> button to add it to the QAT in the right-hand list. OK out. The icon is added to the QAT.