You can create the following pre-request script provided your parameters are defined environment variables. You would need to tweak this example if they are defined in some other fashion.
// Access your env variables like this
var str_1 = environment.variable_1 + environment.variable_2;
// Or get your request parameters
var str_2 = request.data["foo"] + request.data["bar"];
// Use the CryptoJS
var hash = CryptoJS.MD5(str_1 + str_2).toString();
// Set the new environment variable
postman.setEnvironmentVariable('hash', hash);
CryptoJS works because it’s available in Postman (as well as lodash, backbone etc).
Accessing environment variables is easy through the environment object.
Setting environment variables is available through the postman object.
After this pre-request has run you can access the hash variable using the normal {{hash}} shorthand.
Also, you can read here about libraries, variables and properties available in Postman.