Hello,

There are two options to set multiple values to combo box: from Server Side or Client Side.

From Server Side, you could use this example script below.

protected void Page_Load(object sender, EventArgs e)
{
   // WebCombo1.SetMultipleValues([Your Display Member], [Your Value Member]);
   // For Example
   WebCombo1.SetMultipleValues("Anna Trujillo", "ANATR");
}  

From Client Side, you could use this example script below.

function Button1_onclick()
{
   var WebCombo1 = ISGetObject("WebCombo1");
   // WebCombo1.SetMultipleValues([Your Display Member], [Your Value Member]);
   // For Example
   WebCombo1.SetMultipleValues("Maria Anders", "ALFKI");
   WebCombo1.RefreshTextByValueItems();
}

For further information about this topic, please check in WebCombo documentation.

(How-to: Set initial value of multiple selection WebCombo from client side)

(How-to: Set initial value of multiple selection WebCombo from server side)

Hope this helps. Thank you.

Regards,

Hans