How to display text in a collapsed combobox?

Question

WPF

There is a datagrid in which there is a combobox, now you need to double-click on the cell in the grid in which the combobox is located and then select its value.
How to make it so that when the window is initialized, the combobox would already be active and some of the values ​​\u200b\u200bwere already set in it?
Here is combobox

<DataGridComboBoxColumn x:Name="testComboBox" ItemsSource="{Binding category}" Header="Категории" DisplayMemberPath="id_category" SelectedValuePath="name_category"/>

Here's how it fills up now

private void FilltestComboBox()
        {
            addDataAdapter = new MySqlDataAdapter
            {
                SelectCommand = new MySqlCommand()
                {
                    Connection = programQuery,
                    CommandText = "SELECT * FROM category"
                }
            };

            addDataAdapter.Fill(addDataSet, "category");
            DataTable dtEmpName = addDataSet.Tables["category"];
            testComboBox.ItemsSource = ((IListSource)dtEmpName).GetList();
            testComboBox.SelectedValuePath = "id_category";
            testComboBox.DisplayMemberPath = "name_category";

        }