Hi,
You need to include your custom field value in post request, so that it can be process and saved in task metadata.
Go to /libs/cq/inbox/gui/components/inbox/clientlibs/commons/js/inbox.commons.workitem.js and add below line at line number 399. so your new data will be look like this. the 3rd line have my custom type stepTitle.
data[("stepTitle-" + taskId)] = $("[name=stepTitle]", "#workitemCompletionDialog").val();
After adding your code it will have all 3 values.
data[(postDataSelector + taskId)] = coralNextStepSelect.value;
data[("comment-" + taskId)] = $("[name=comment]", "#workitemCompletionDialog").val();
data[("stepTitle-" + taskId)] = $("[name=stepTitle]", "#workitemCompletionDialog").val();


