need help on JSON request and response on UI only

I have a server model where, if we send JSON request, server replays
back with JSON response. Now I need to develop UI part for this
project using GWT (no need to develop server side as it has been
already developed).Can you please suggest me how I send and receive
JSON data from GWT front end.
To brief, server receives JSON request and replays back with JSON
response. Now I need t create UI where I send JSON request and receive
JSON response.
I believe Making Cross-Site Requests is one option. Not sure are there
any other ways to do this.(I just need to send JSON request and
receive response from server without worrying about server code)

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