If you have access to the server then Ralph's approach is by far the best, as it translates well to all clients, but it's tough when you're on a SaaS instance.

The following JavaScript snippet creates a new approval via the undocumented WorkItemProxy - use at your own risk and peril!!! If you don't know what you're doing then Bad Things(tm) can happen!

The two Utils functions are used to check whether the approval already exists, and to retrieve a list of internal user ids that need to be added to the approval - in my Utils function this is done based on their assigned role in the team associated with the Filed Against category.

/*******************************************************************************
 * CLM workitem customisation scripts
 * (c) Copyright Davyd Norris 2019. All Rights Reserved.
 *
 ******************************************************************************/

dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");
dojo.require("og.Utilities");

(function() {
  var WorkItemAttributes = com.ibm.team.workitem.api.common.WorkItemAttributes;
  var Utils = og.Utilities;

  var createApproval = function(workItem, title, filedAgainst, role, type) {
    if (Utils.hasApprovalBeenCreated(title)) {
      return;
    }
   
    var dueDate = new Date();
    var newApproval = {
      cumulativeState : "com.ibm.team.workitem.approvalState.pending",
      approvalType : 'com.ibm.team.workitem.approvalType.' + type,
      dueDate : dueDate.setDate(dueDate.getDate() + 2),
      name : title,
      approvals : [],
      _isNew : true
    };

    Utils.getCategoryList('projects')
    .then(function(projects) {
      var teamArea = projects.find(function(item) {
        return item.id === filedAgainst;
      }).teamArea;
      return Utils.getTeamMembersWithRole(teamArea, role);
    })
    .then(function(members) {
      for (var i = 0; i < members.length; i++) {
        newApproval.approvals.push({
          approver : {
            itemId : members[i]
          },
          state : 'com.ibm.team.workitem.approvalState.pending',
          _isNew : true
        });
      }
     
      var newId = workItem.getProxy().getValue( {path :[ 'approvals' ],defaultValue:[]} ).length;
      workItem.getProxy().setValue({
        ignoreChange: false,
        notifyEvenIfUnchanged:false,
        path: ['approvals', newId],
        value: newApproval
      });

      workItem.getProxy().storeWorkItem({
        self : this,
        operationMsg : "Saving",
        applyDelta : true,
        onSuccess : function(param) {
          console.log("Approval Saved");
        }
      });
    });
  };

})();

This is how you would use the function somewhere in your attribute customisation script (this one creates a Review type of approval):

var projId = workItem.getValue(WorkItemAttributes.FILED_AGAINST);
createApproval(workItem, 'Project Director Assessment', projId, 'Project Director', 'review');