The parameter input_filter follows a format specified by a particular definition in each API.

For instance, as you can see in the image below, for "/assets/accounts/{accountId}/transactions/{input_filter}", the input_filter follows the definition Asset_Accounts_Transactions_Input, in which you can also find an example.

For your convenience, here is an example for input_filter :

{
      "fromDate": "2015-1-9",
      "toDate": "2015-4-9",
      "fromRow": "",
      "pageSize": 20,
      "lastBalance": 0.0
}