Missing parameter: client_id for lwc

Im working on a lwc where id like to embed a signing process in iframe, I was able to successfully retrieve sign URL, but when I try to use it on iframe I got following error, at which step do I need to set my client id?
So far I used this with client id in request body

String apiUrl = 'https://api.hellosign.com/v3/signature_request/create_embedded_with_template';
and 

request.setEndpoint('https://api.hellosign.com/v3/embedded/sign_url/' + signatureId);

with just signature id in request body

Also I have a problem setting body for my request, I used this class to generate json string, but when I use this approach I get error({"error":{"error_msg":"No template_id or template_ids specified","error_path":"template_id","error_name":"bad_request"}})

HelloSignJSON.Signing_options so = new HelloSignJSON.Signing_options();

so.default_type = 'draw';

so.draw = true;

so.phone = true;

so.type = true;

so.upload = true;

HelloSignJSON.Signers signer = new HelloSignJSON.Signers();

signer.role = 'Lead.Customer';

signer.name = 'Rostyslav';

signer.email_address = 'popov.rst@gmail.com';

List<HelloSignJSON.Signers> signers = new List<HelloSignJSON.Signers>{signer};

HelloSignJSON hsj = new HelloSignJSON();

hsj.template_ids = new List<String>{'90c43afee504f24a2dcd67ab7991f8f3f5900ef3'};

hsj.client_id = 'CLIENT_ID';

hsj.subject = 'Embedded Subject';

hsj.message = 'Embedded message';

hsj.signing_options = so;

hsj.test_mode = true;

hsj.signers = signers;

String requestBody = JSON.serialize(hsj);