FeeArrangementService.createContingencyFeeMatterFeeArrangement(
   @feeArrangementRecordId,
   @matterRecordId,
   @contingencyPercentage,
   @defaultForFees,
   @defaultForExpenses
);

@feeArrangementRecordId
The fee arrangement record's id value using which the fixed matter fee arrangement is to be created.

@matterRecordId
The matter record's id value for which the fixed matter fee arrangement is to be created.

@contingencyPercentage
Your finalized contingency percentage offered to client.

@defaultForFees
To mark this matter fee arrangement as default when entering time for this matter.

@defaultForExpenses
To mark this matter fee arrangement as default when entering expenses for this matter.

@recurrence
Provide how often the client is to be charged for this fees.
(Accepted values are: One Time, Monthly, Quarterly, Annually)

@startDate
If your recurrence is other than One Time then provide the start date for it.

@endDate
If your recurrence is other than One Time then provide the end date for it.

The function will return list of sObjects of matter fee arrangement type.

advpm__Matter_Fee_Arrangement__c result = FeeArrangementService.createContingencyFeeMatterFeeArrangement(
  'a0A9000000RJhKTE',
  'a05900000051CpN',
  35,
  false,
  false
);
System.debug('Matter Fee Arrangement created : ' + result.Name);