Client signups and onboarding is handled by the Client Management service group. In there you can find the endpoint GET /cm/v1/signup/status which lets you select specific client (by ClientKey) and provide a response with details on the onboarding status:
{
"ClientId": "7856478",
"ClientKey": "d2-PzdC5wIPy1284wD-rRA==",
"Message": "Onboarding request complete. Account and client keys have been created",
"OnboardingState": "ClientOnboarded"
}
Where the field OnboardingState holds the current onboarding status.
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