The steps are roughly:
1) Create the data object in the Data Layer tab. Set it as a UDO object
2) Add a Set Data Values extension.
This will assign the value from the textbox into the Tealium variable which will then be available for all tags
You might need to fiddle with the scope to make sure it fires at the right time.
If you want to read the value when the user changes it, you can add an event handler extension with a similar set up, but with the scope being blur on the appropriate textbox.