You can use the SOAP API to create a prepayment charge. WSDL version 114+ is required for using the Prepaid with Drawdown feature.

Sample SOAP API request: Create a monthly recurring prepayment charge.

<ns1:create>
 <ns1:zObjects xsi:type="ns2:ProductRatePlanCharge">
  <ns2:AccountingCode>Accounts Receivable</ns2:AccountingCode>
  <ns2:BillingPeriod>Month</ns2:BillingPeriod>
  <ns2:BillingPeriodAlignment>AlignToCharge</ns2:BillingPeriodAlignment>
  <ns2:ChargeModel>Flat Fee Pricing</ns2:ChargeModel>
  <ns2:ChargeType>Recurring</ns2:ChargeType>
  <ns2:Name>Monthly Plan</ns2:Name>
  <ns2:ProductRatePlanChargeTierData>
   <ns1:ProductRatePlanChargeTier xsi:type="ns2:ProductRatePlanChargeTier">
    <ns2:Active>true</ns2:Active>
    <ns2:Currency>USD</ns2:Currency>     
    <ns2:Price>20</ns2:Price>
   </ns1:ProductRatePlanChargeTier>
  </ns2:ProductRatePlanChargeTierData>        
  <ns2:ProductRatePlanId>2c92c0f87a85be74017a88ee747862a8</ns2:ProductRatePlanId>
  <ns2:TriggerEvent>ContractEffective</ns2:TriggerEvent>
//Fields related to prepayment charge
           <ns2:IsPrepaid>true</ns2:IsPrepaid>
           <ns2:PrepaidOperationType>topup</ns2:PrepaidOperationType>
           <ns2:PrepaidQuantity>1</ns2:PrepaidQuantity>
           <ns2:PrepaidUom>Million calls</ns2:PrepaidUOM>
           <ns2:ValidityPeriodType>MONTH</ns2:ValidityPeriodType>
 </ns1:zObjects>
</ns1:create>