Add existing_agreement parameter to Intrum service onboarding

The PUT /v2/services/intrum/receivables endpoint now supports a new optional boolean parameter existing_agreement. This parameter streamlines the onboarding process for companies that already have an established agreement with Intrum.

Setting existing_agreement to true, will skip the standard service activation workflow. This allows companies to proceed directly to activating their service using their existing client_code via the PATCH /v2/services/intrum/receivables endpoint.

The parameter accepts:

• true - company already has an existing agreement with Intrum
• false or omitted - new agreement, standard onboarding process will be followed