Bearer token example
If your API requires you to use a bearer token, you might want to use a script like this in the Post Response scripting section:
javascript
bru.setVar("token", res.body.access_token);
With this method, you can then use the variable in other requests. Just select Bearer Token in the Auth tab and insert your variable.
String splitting
If the authentication API does not deliver a JSON response but rather a singe string that looks like this "Bearer FOOBAR"
you might want to split the string with a little something like this:
javascript
bru.setVar("token", res.body.split(" ")[1]);