Adding comment to to record by simply create WORKLOG mbo through the rest Api is not possible.
Such action results in Error 400: comlog#ownerNull.
Right way to do this is adding WORKLOG from parent record, using custom object structure (which has to have relation to WORKLOG enabled).
1. Choose (or create) Object Structure for Integration, which will have relation to WORKLOG object.
2. Fetch record using that object structure
3. Add workload in form of attributes list:
(…) where index will be number of existing comments + 1