Json to CSV select second level columns - powershell

Question

I have a json in the below format, where some columns are at 2nd level. I want to the column at the second level to be at the first level in CSV
{
"ApiKey": "123",
"BasicDetails": {
"ClientID": "1234",
"CompanyName": "C1",
"ContactName": "",
"EmailAddress": "",
"Country": "United Kingdom",
"TimeZone": "(GMT+00:00)"
}}
Required Output is:
APIKey, ClientID, CompanyName, ContactName, EmailAddress,Country,Timezone
123,1234,C1,,,United Kingdom,(GMT+00:00)
I have tried:
(GET-Content F:/my.json -RAW | ConvertFrom-Json)|Select Columns|Export-CSV F:/my.csv
EDIT
Actual Json:
{
"ApiKey": "123",
"BasicDetails": {
"ClientID": "1234",
"CompanyName": "C1",
"ContactName": "",
"EmailAddress": "",
"Country": "United Kingdom",
"TimeZone": "(GMT+00:00)"
}
"BillingDetails": {
"CurrentTier": "0 - 500",
"CurrentMonthlyRate": 9.0000,
"MarkupPercentage": 0,
"MonthlyScheme": "Basic",
"Currency": "USD",
"ClientPays": false
}}
I need Belling Detail columns, Basic Details column and API key at one level

Answer 1

This should work for a all JSON files with a depth of two (which you requested):
$tempObject = #{}
(GET-Content F:/my.json -RAW | convertfrom-json).PsObject.Properties | ForEach-Object {
if ($_.TypeNameOfValue -eq 'System.Management.Automation.PSCustomObject')
{
$_.Value.PsObject.Properties | ForEach-Object {
$tempObject += #{$_.Name = $_.Value}
}
}
else
{
$tempObject += #{$_.Name = $_.Value}
}
}
[PsCustomObject]$tempObject | Export-CSV F:/my.csv

Answer 2

Tricky question. Depends very much if you are looking for a solution for your specific case or a more general approach.
For the first, go with Martin Brandl's answer.
Else, there is also ConvertTo-FlatObject:
https://gallery.technet.microsoft.com/ConvertTo-FlatObject-396a6e0a