Public Function CreateBankRelationship()

    Dim oFin As CFinancial2
    Set oFin = New CFinancial2
    oFin.Init REApplication.SessionContext
    With oFin
         ''What constituent are you adding the bank to?
        .Fields(FINANCIAL_fld_CONSTIT_ID) = 755
         ''What bank are you adding? Must already exist.
        .Fields(FINANCIAL_fld_BANK_ID) = 21
        .Fields(FINANCIAL2_fld_ACCOUNT_NO) = "123456"
        .Fields(FINANCIAL2_fld_IS_PRIMARY) = True
    End With
    Set oFin = Nothing

End Function

If the bank record does not already exist, please review the following article:
How to write code to create a bank record​
Note: The Constit_ID field in the Constituent_Bank table is the Records_ID from the Constituent table. Also, when creating a gift the Constituent_Bank.ID is used instead of the Bank.ID.

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