In the "Data" field, you will want to escape the quotes characters (").
This would give something more akin to :
"{""username"":""u2"",""firstname"":""sha2"",""lastname"":""last2"",""pasword"":""pass123""}"
And you may even use the system expression URLEncode, to make sure the characters will be made to be used/displayed in a URL :
urlEncode("{""username"":""u2"",""firstname"":""sha2"",""lastname"":""last2"",""pasword"":""pass123""}")
I haven't tested it, but it should work.
Also in your JSON "pasword" is possibly a typo, unless your server-side script is looking for the keyword "pasword" and not "password".