I believe writeuser is the system user and you are making post request that's why you are getting 405 in browser.
Go to configurations in felix http://servername:port/system/console/configMgr
Search for "user mapper service " or writeuser
you will see an entry like mentioned below
if it is not there then add one.
Please search for writeuser in http://aemhost:port/security/users.html like mentioned below -
if it is not there then create system user named: writeuser and add requisite permission to writeuser .
Hope this will help.