custom product options with shopify admin API

[Ahsan3]

Hi

I need to add multiple options with more than 100 variants and I know that I need to use an app.

But I need to enter or POST the products and their variants using a third-party API. 

How can I add variants to the app using Shopify Admin API?

Or is there any way that I can add variants directly into that app using any POST API?

Kindly please try to respond as soon as possible.