I am working on (built-in web apis) provided by whatsapp business api. As a newbie in .net core razor pages and web apis. I want to know how can I get access to the body of the post request api. Take an example below for sending a message

Post: {URL}/v1/messages Request Body: "to": "", "message_type:" "message_text:" "recipient_type: "individual | group"" 

How can I make a call to the builtin api and access the body parts of it? Ofcourse, we as a developer can use postman for checking the working of api. But take this as a client and for the client we have some fields like

To: Message: 

How can take these fields and put it into the api call body and then when the user click on the send, the api call works and shows whatever we want to show the user for example a model with send successfully etc.