For more details on customizing VA number, go here.

"bank_transfer":{
  "bank": "bca",
  "va_number": "111111",
  "free_text": {
       "inquiry": [
             {
                 "id": "Free Text ID Free Text ID Free Text ID",
                 "en": "Free Text EN Free Text EN Free Text EN"
             }
       ],
       "payment": [
             {
                 "id": "Free Text ID Free Text ID Free Text ID",
                 "en": "Free Text EN Free Text EN Free Text EN"
             }
       ]
  },
  "bca": {
    "sub_company_code": "00000"
  }
}

"bank_transfer":{
  "bank": "permata",
  "va_number": "111111",
  "permata": {
    "recipient_name": "SUDARSONO"
  }
}

"bank_transfer":{
     "bank": "bni",
     "va_number": "111111"
  }

JSON Attribute	Description	Type	Required
bank	Bank name which processes bank transfer transaction. Possible values are `permata`, `bni`, `bri`, and `bca`.	String(255)	Required
va_number	Custom VA number assigned by you. If shorter than minimum then Midtrans will add trailing 0s as most significant bits. If longer than maximum then Midtrans will trim the remaining least significant bits. BCA VA: Accepts 6-18 digits Permata VA: Accepts 12 digits. BNI VA: Accepts 8 (For 5 digits Client ID) or 12 digits (For 3 digits Client ID). BRI VA: Accepts 18 digits. CIMB VA: Accepts 12 digits.	String(255)	Optional
free_text	List of free texts. Note: Right now only used for BCA VA.	JSON Object Array	Optional
bca	Specific parameters for BCA VA.	JSON Object	Optional
permata	Specific parameters for Permata VA.	JSON Object	Optional

Free Text Object

JSON Attribute	Description	Type	Required
inquiry	Free texts shown during inquiry.	JSON Object Array(10)	Optional
payment	Free texts shown during payment.	JSON Object Array(10)	Optional

Inquiry/Payment Object

JSON Attribute	Description	Type	Required
id	Free text message in Bahasa Indonesia.	String(50)	Required
en	Free text message in English.	String(50)	Required

BCA VA Object

JSON Attribute	Description	Type	Required
sub_company_code	BCA sub company code directed for this transactions. Note: Default is `00000`.	String	Optional

Permata VA Object

JSON Attribute	Description	Type	Required
recipient_name	Recipient name shown on the payment details. It is shown as 20 characters uppercase string. Note: Default is merchant name.	String	Optional

BNI VA Object

JSON Attribute	Description	Type	Required
bank	The name of the bank which processes the transaction using Bank Transfer payment method.	String(255)	Required
va_number	Custom VA number assigned by merchant. Min length 1, max length 8	String(255)	Optional