How to change the price of a product in Magente when adding it to the cart via the API?

Question

Magento

If you change the price not through the API, then this is done through the Observer, here it is analyzed in detail - https://store.magenest.com/blog/set-custom-price-p...
But the problem is that when using the Observer API does not work. Tell me how to be.
Endpoints rest/V1/guest-carts/..../items or /rest/V1/carts/mine/items