используйте sendkeys:
comboBoxElement1.SendKeys("name of the item");
ОБНОВЛЕНИЕ
comboBox.Click();
string xPathListItem = $"//Text[contains(@Name, '{dateTom}')]/preceding::Custom[1]/ComboBox/ListItem[1]"; //xPath of your item in combobox
elem = (WindowsElement)window.FindElementByXPath(xPathListItem);
app.DoubleClick(elem);
вот мой метод DoubleClick:
public void DoubleClick(WindowsElement elem)
{
session.Mouse.MouseMove(elem.Coordinates);
session.Mouse.DoubleClick(null);
}