Add cart items via API to storefront cart

Hi, I am building an Angular integration in Shopware 6, the whole store relies on the storefront theme, but the JS application can add products via the store-api as well. Now how can I add the item to the storefront cart?
I post the data to the /store-api/checkout/cart/line-item API and even with context-token which I fetched from the /store-api/context API before, it doesn’t work. Is there any way I can add items to the storefront cart with the current session via the API? Or maybe I can get the current context output somewhere to then take the token and update the current cart?