SQLMAP - POST JSON data as body

Hi I'm trying to do a SQL injection in a login form. With BurpSuite I intercept the request:

POST /xxxx/web/Login HTTP/1.1 Host: 10.0.0.70:42020 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:49.0) Gecko/20100101 Firefox/49.0 Accept: application/json, text/plain, */* Accept-Language: it-IT,it;q=0.8,en-US;q=0.5,en;q=0.3 Accept-Encoding: gzip, deflate Content-Type: application/json Referer: http://xxxxxx.com/xxxxxx/ Content-Length: 44 origin: http://xxxxx.com Connection: close
sqlmap -u requestFile 

(where requestFile is the content of the request intercepted with Burp) sqlmap can't find the injectable field that is password. I'm sure it's injectable cause if as password I input:

' OR 1=1; -- - 

I can login with every username I insert Tried also with:

but no luck. What I'm doing wrong? P.S. In every POST or PUT request made with browser, a preflight request with method OPTIONS is made. It's made that to bypass CORS problems. Could it affects request made by sqlmap?