TextBox
As mentioned here (
Drag&drop from an external source) the last parameter of dragStart() specifies the event text. You should modify your onmousedown attribute as follows:
onmousedown='return DayPilotCalendar.dragStart(this, 60*30, "123", this.value);'
If you want the TextBox to be removed after drop, keep the first "this", otherwise replace it with "null" (without quotes). "this.value" refers to the value of <input> element (that's the resulting HTML of TextBox).
GridView cell
To be able to drag from a GridView, you should use a TemplateField to render custom HTML. One of the ways to do it is here:
<asp:TemplateField HeaderText="Item">
<ItemTemplate>
<div onmousedown='return DayPilotCalendar.dragStart(this, <%# Eval("DURATIONINSEC") %>, <%# Eval("ID") %>, <%# Eval("TEXT") %>);'>
<asp:Label ID="LabelItem" runat="server" Text='<%# Eval("TEXT") %>'></asp:Label></div>
</ItemTemplate>
</asp:TemplateField>
There might be some typos, especially in the double/single quotes combination, but you should get the idea. Let me know if there is any problem with this solution.