As far as I can tell you cannot use HttpClient get to send a body. You could use the query at least for it to be idiomatic. (Or you can try to force the issue somehow).
Lets say you have an array with your criteria:
const criteria = [ {a: 25}, {b: 23} ];
http.get(url + '/?criteria="+ encodeURIComponent( JSON.stringify(criteria)));
Sending a body in a GET request is a violation of some RFC standard, and even though it might work you”re bound to summon some arcane demons.