Boards the merchant onto an acquirer synchronously and returns the
resulting connection with 201. There is no pending state.

You name an acquirer (your bank relationship) and the
acquirer-assigned merchant_id (MID), plus the
terminal_id (TID) when the acquirer needs one. Fat Zebra validates the MID/TID and currencies
against its config and boards the merchant onto every processor behind
that acquirer that can carry a requested currency — you never name a
processor. Omit currencies to board everything the acquirer supports.

Boarding is idempotent: re-posting the same acquirer returns the
existing connection rather than creating a duplicate.

Failures are reported inline:

• 422 validation_error — an unsupported currency, a missing
  merchant_id, or no
  processor supports the acquirer for the requested currencies. The
  offending fields are listed under error.fields.
• 422 processor_error — an underlying processor board was rejected or
  errored upstream.
• 404 not_found — the acquirer is unknown or not available to this
  partner.