Is it possible to insert new record using custom Javascript code from dynamics 365 portal website?

Hello,

I have requirement where customer (portal user) can insert record from dynamics 365 portal website. (without using Entity form or Web form). Where I will have my custom html form and javascript. Data needs to be submitted to dynamics 365 using javascript. 

Is it possible to do so? if yes that how can we manage authentication?

I have tried code given here - https://msdn.microsoft.com/en-in/library/gg334279.aspx but I am getting unauthorized error while accessing 

<clientURL> + "/api/data/v8.1/accounts

Regards,

Ashish