How can I bind textbox in the insert template to external datasource?

I have a DetailsView control using templates. When in Insert mode for one field ("ItemSequence")I want to prompt users with a data value (next seq#). DetailsView is bound to ITEMS datasource, running the Get_Item stored procedure returning record with 16 fields in it. Users may leave that value to be inserted or they can change it for whatever they need. I also have a datasource running a stored procedure Get_Next_Seq returning that next_seq# value. 

• atou rename database when detach /attach
• Attach Database with Rename(Attash as)
• Are Microsoft® SQL Server® 2014 Feature Pack components compatible with SQL Server 2012
• [DTM_10004] The DTM process terminated unexpectedly.
• create database permission denied in database master
• Microsoft Sql Server 2014 Express and AdventureWorks 2014 Script
• SQL Server Express - Installation Sequence
• Dumb question - SQL Server Express 2014 - where is my "SQL" interface?
• Cannot Generate SSPI Context
• SQL Express usage
• Cannot Export from Sql Server Express 2014 to Access 2016
• Managing SQL 2014 Standard
• SQL 2014 Express and WSUS on Windows 2012 R2
• sql server Migration error
• Install SQL Server 2014 Express in Windows 10
• Suddenly lost ability for client connections to SQL Server Express
• MS SQL Server 2014 fails creating a backup
• Linked server taking time
• Connection pool exceeded error
• Not able to install/Uninstall SQL Server Express 2014