MATLAB: Can’t figure out what this API want’s me to use as data when using matlab for a POST request

api encryption

If you look at this 2 links:
I have been trying for 2 days now to structure my code so I can authenticate there, but it's just wrong.
What exactly should I put in the data param for the HMAC encryption? How does the body Matlab sends in a webread POST request look? As I said, Im getting nowhere with this.
import matlab.net.*
import matlab.net.http.*
URI = matlab.net.URI('https://testnet.bitmex.com/api/v1/order');
U = '?symbol=XBTUSD&side=Sell&orderQty=30&clOrdLinkID=SoL&ordType=Market&contingencyType=OneCancelsTheOther';
Meth = matlab.net.http.RequestMethod.POST;
Req = matlab.net.http.RequestMessage;
Req.Method = Meth;
Body = matlab.net.http.MessageBody(U);
Req.Body = Body;
ApiExpires = datetime('now','format','yyyy-MM-dd''T''HH:mm:ss''Z') + days(5);
ApiExpires = posixtime(ApiExpires);
ApiExpires = round(ApiExpires);
ApiExpires = num2str(ApiExpires);
ApiSecret = 'Tu9CNn_04cQxvjdgGm3MBV7KsqxzkNMAw_3LtUuLUhqC--iX';
Com = complete(Req,URI);
Path = '/api/v1/order';
Verb = 'POST';
EncData = strcat(Verb,Path,ApiExpires,'?symbol=XBTUSD&side=Sell&orderQty=30&clOrdLinkID=SoL&ordType=Market&contingencyType=OneCancelsTheOther');
signature = HMAC(ApiSecret,EncData,'SHA-256');
signature = lower(signature);
ApiSignature = matlab.net.http.HeaderField('api-signature',signature);
ApiKey2 = matlab.net.http.HeaderField('api-key','C8VW3PArKaNoh04CE103hucF');
ApiExpires2 = matlab.net.http.HeaderField('api-expires',ApiExpires);
Header = [ApiExpires2,ApiKey2,ApiSignature];
Req.Header = Header;
Why is this unauthorized when doing send(Req,URI)? (Testnet so do what you want with the keys)

Best Answer

Solved it by adding:
ContentType = matlab.net.http.HeaderField('Content-Type','application/x-www-form-urlencoded');
Header = [ContentType,ApiExpires2,ApiKey2,ApiSignature];