The VB code you've showed me looks to be correct. If the RadTextBox's value is empty at this event (InsertCommand), most likely the Controls is not being bound to the template.
Can you double check and ensure that RadTextBox's Text property is bound as shown in the following example?
<%# Eval("SomeField") %>
<telerik:RadTextBox ID="RadTextBox1" runat="server" Text='<%# Bind("SomeField") %>'></telerik:RadTextBox>
If this is done and the value is still not present at InsertCommand, please share the RadGrid markup and code behind code so that I can check out how the entire configuration looks like and advise you based on that.
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