If you Google flutter query parameter then the first link in the top result is to a StackOverflow post that answers your question. I strongly recommend Googling any question you have. It's what professional developers do, I believe ;-)
Here's the code in that answer:
final queryParameters = {
'param1': 'one',
'param2': 'two',
};
final uri =
Uri.https('
www.myurl.com', '/api/v1/test/${
widget.pk}', queryParameters);
final response = await http.get(uri, headers: {
HttpHeaders.authorizationHeader: 'Token $token',
HttpHeaders.contentTypeHeader: 'application/json',
});