Add and/or choose the repayment method for borrower.

If payment method is "DIRECT_DEBIT" then the account parameter must not be null.

If is_default_repayment_method is true, then the repayment method becomes the borrower's default
repayment method so that upcmoing loan requests will use this payment method and account as a default.

Please note: that the account defined in the loan_to attribute of created loans is left
unchanged. If you want to change the destination account of a loan, you must cancel it and
request a new loan.

If apply_to_all_active_loans is true, then the repayment method is used for upcoming payments of
all loans that are in TO_REPAY or TO_REPAY_FEES statuses.