Our online demo shows how to set the value of a CheckBoxList - through the Value property.
<RadzenCheckBoxList @bind-Value="values" TValue="int">
<Items>
<RadzenCheckBoxListItem Text="Orders" Value="1" />
<RadzenCheckBoxListItem Text="Employees" Value="2" />
<RadzenCheckBoxListItem Text="Customers" Value="3" />
</Items>
</RadzenCheckBoxList>
@code {
// The Value of the option(s) that will be checked
IEnumerable<int> values = new int[] { 1 };
}