Get Engagement Details

Returns engagement information for the anonymous acceptance page.

Requires a valid engagement GUID that has not yet been accepted or rejected.
Returns tenant details, content blocks, and engagement summary.

Optional Query Parameters:

• callbackUrl: The URL to redirect the user back to after completing external steps (Direct Debit mandate setup, AML identity verification).
  Used by both DD and AML providers as a return URL after their hosted flows complete.
• preview: When true, skips updating LastViewed timestamp and Sent→Viewed status transition.
  Used by practice staff to preview the acceptance page without affecting engagement tracking.