Unfortunately, R2018a does not support remembering the predefined path. Remembering the report path during MATLAB sesssion was introduced since R2019a.
As a workaround, you can try to use the custom facing APIs to see whether it will satisfy your needs:
loadedReqSet = slreq.load(filepathOrNameOfSavedReqSet);
reqOptions = slreq.getReportOptions;
reqOptions.reportPath = fullfile(yourPredefinedPath, [reqSetName, '.docx']);
You can create a shortcut to call this script to generate report without reinvoking the report option dialog every time.
Hope this helps.