Use the event on the textbox (onChange) and let it execute a nanoflow.
In this nanoflow, check if the user entered the ‘last number'.
In that case, use the ‘setFocus’ action from the webactions module to set the focus of the user to the next input field.
This module is added in the default boilerplate template, or you can add it manually from here: https://docs.mendix.com/appstore/modules/web-actions/
Hope this helps.