How to implement synchronous callback

There are two servers, severA, severB, user C

User C accesses serverA, and then POSTs a form to serverB. At this time, the page the user sees is the page provided by serverB. After serverB processes the business, POST submits the processing information to serverA, and the user page returns to serverA.

Question: How does serverB implement this post callback? Does serverB return an html structure to user C, including a form structure? After user C gets the html, he immediately submits the form?

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There are two servers, severA, severB, user C

User C accesses serverA, and then POSTs a form to serverB. At this time, the page the user sees is the page provided by serverB. After serverB processes the business, POST submits the processing information to serverA, and the user page returns to serverA.

Question: How does serverB implement this post callback? Does serverB return an html structure to user C, including a form structure? After user C gets the html, he immediately submits the form?

Is it ok for serverB curl to request serverA? ServerB does an internal forwarding to request serverb. Is it ok?

I feel like you have already answered: "serverB returns an html structure to user C, including a form structure. After user C gets the html, he immediately submits the form."

I feel that the application scenario mentioned by the poster is a bit like the Alipay merchant payment scenario. Post data from your own page and then jump to the Alipay payment page
Then after the payment is successful, Alipay will jump to your page based on the return_url you passed.
The poster can simulate the business logic processing process of Alipay