Am I right in understanding that the Trigger that gets called will call your stored procedure?

If this is the case, all you need to do is get the identity (Good practise is to always have a Identity Column anyway) that was insterted into the mydata table and pass it to the Stored Procedure. Then you can build a SQL String that you can modify with Variables.

Regarding the the Type in the timestamp column, you can use varchar or nvarchar and cast your value to a string?

Or if you want to, you could use sql_variant.

The choice is up to you.

Let me know if this was helpful!

Pierre