Evaluate trigger from approval process

The trigger is not automatically invoked upon the record approval unless there is a field updated during the approval step. To address this, create a checkbox field that should be marked 'true' once record is approved.

Steps could be as follows:

• Create a checkbox field to update when its Approved
• Update this checkbox to true in Approval actions.
• Compare in trigger and have this in 'IF' Condition whether 'Check_ Box == True'  

Sample Code: 

trigger CreateRecord on Case (after insert) {

        for (Case c : Trigger.new) {
            if (Check_ Box__c = true) {
            // Define parameters to be used in calling Apex Class
            String objectType ='Case';  
            String objectId = c.id;
            String projectKey = 'SFDC'; 
            String issueType = '1';     
            // Calls the actual callout to create another record.
            SFDCSampleWebserviceCalloutCreate.createRecord(PARAM1, PARAM2, PARAM3);
        }
    }