Reach JSON object by ignoring what comes before

Allright, this is what I have in a variable.

[SXWE] Nickname : test
{
  "Message": "test",
  "UserId": 50,
  "Username": "Nickname"
}

I want to check if this json object exists by using hasOwnproperty. The issue here is that I have something before the object itself and it seems like it does not find the key of the Object because it is getting blocked at the first line

via Nox