You can store anything like that in a string.
What I think the problem is here is that to define a constant string you do it like this:
string x = "This is my string";
Because your json has double-quotes in them, you need to escape them:
var json="{\"access_token\":\"i7aJLVH_SKIkNF1BI1zQ==\",\"scope\":\"UtilityNetworkAuth\",\"expires_in\":57599,\"refresh_token\":\"jMwIe1_33C1b2tFDahvmA==\"}";
This can be painful for constants, so I usually just stick the string in a resource file.