Set default value of ComboBox in grouping DataGrid

I have this,

Now, I need to set the default value of my ComboBox, (i.e., to the first value of my database table). I have tried to do this manually, ComboScanner.Text = "", however, I get an error saying:

Could not find 'ComboScanner'.

I am wondering if this is because I am using a DataTemplate, if not, how can I solve this?

Have you tried setting your ComboBox's SelectedIndex = 0?

<ComboBox Name="ComboScanner" SelectedIndex="0" ... />