In a browser's REST client please try to POST:
https://rally1.rallydev.com/slm/webservice/v2.0/defectsuite/22222/defects/add?key=abc123...
using this request body:
{
"CollectionItems": [
{"_ref": "/defect/3333"}
]
}
For more info on how v2.0 deals with collections please see Collection section in WS API documentation.
Also, notice that authorization key needs to be generated first:
https://rally1.rallydev.com/slm/webservice/v2.0/security/authorize
You will GET back a string
{"OperationResult": {"_rallyAPIMajor": "2", "_rallyAPIMinor": "0", "Errors": [], "Warnings": [], "SecurityToken": "abc123..."}}