How to use the node-opcua-service-endpoints.UserIdentityTokenType.USERNAME function in node-opcua-service-endpoints

// assert(endpoint instanceof EndpointDescription);
    assert(userName === null || typeof userName === "string");
    assert(password === null || typeof password === "string");
    const endpoint_desc = session.endpoint;
    assert(endpoint_desc instanceof EndpointDescription);

    /**
     * OPC Unified Architecture 1.0.4:  Part 4 155
     * Each UserIdentityToken allowed by an Endpoint shall have a UserTokenPolicy specified in the
     * EndpointDescription. The UserTokenPolicy specifies what SecurityPolicy to use when encrypting
     * or signing. If this SecurityPolicy is omitted then the Client uses the SecurityPolicy in the
     * EndpointDescription. If the matching SecurityPolicy is set to None then no encryption or signature
     * is required.
     *
     */
    const userTokenPolicy = findUserTokenPolicy(endpoint_desc, UserIdentityTokenType.USERNAME);

    // istanbul ignore next
    if (!userTokenPolicy) {
        throw new Error("Cannot find USERNAME user token policy in end point description");
    }

    let securityPolicy = fromURI(userTokenPolicy.securityPolicyUri);

    // if the security policy is not specified we use the session security policy
    if (securityPolicy === SecurityPolicy.Invalid) {
        securityPolicy = session._client._secureChannel.securityPolicy;
        assert(securityPolicy);
    }

    let identityToken;
    let serverCertificate = session.serverCertificate;