Create list item with people lookup field

createlistitem.cs

var expenseItem = new ListItem
{
    Fields = new FieldValueSet
    {
        AdditionalData = new Dictionary<string, object>
        {
            {"Title", expense.Title},
            {"Zeitpunkt", expense.Date},
            {"Betrag", expense.Amount},
            {"MitarbeiterLookupId", await GetUserLookupId(expense.User)}
        }
    }
};

var createdItem = await _graph
    .Sites[_configuration["SiteId"]]
    .Lists["Spesen"]
    .Items.Request().AddAsync(expenseItem, cancellationToken);
    
   
private async Task<string> GetUserLookupId(string user)
{
    var hiddenUserListId = (await _graph
        .Sites[_configuration["SiteId"]]
        .Lists.Request().Filter("displayName eq 'Benutzerinformationsliste'")
        .GetAsync())[0].Id;

    var userLookupId = (await _graph
        .Sites[_configuration["SiteId"]]
        .Lists[hiddenUserListId]
        .Items
        .Request()
        .Header("Prefer", "HonorNonIndexedQueriesWarningMayFailRandomly")
        .Filter($"fields/UserName eq '{user}'")
        .GetAsync())[0].Id;

    return userLookupId;
}