@CanIgnoreReturnValue
    public SingleSignOnParticipationRequest attribute(final String key, final Object value) {
        this.attributes.put(key, value);
        return this;
    }
Put attribute.

@param key   the key
@param value the value
@return the single sign on participation request
api/cas-server-core-api-webflow/src/main/java/org/apereo/cas/web/flow/SingleSignOnParticipationRequest.java