Sample apex code to make a request to Walmart API service and generate the signature for the WM_SEC.AUTH_SIGNATURE header from Salesforce

Walmart.cls

/**
 * Edgar Moran 
 * March 2022
 */
public with sharing class Walmart {

    @InvocableMethod(label='Send Message')    
    public static List<String> sendMessage(List<String> message) {

        String consumerId       ='CONSUMER_ID';
        String baseURL          ='SERVICE_BASE_URL';

        /**
         * String encodedPrivateKey [You need to convert your .pem private key  to PK8 format using this command: openssl pkcs8 -topk8 -nocrypt -in key.pem -outform PEM
        *  you need to paste the text between -----BEGIN PRIVATE KEY----- and -----END PRIVATE KEY-----]    
         */
        String privateEncodedKey='CONVERTED_KEY_PK8';
        String httpMethod       = 'REQUEST_METHOD';
        String privateKeyVersion = '1';
    
        String timestamp        =  String.valueOf(DateTime.now().getTime());
        String stringToSign     = consumerId + '\n' + timestamp + '\n' + privateKeyVersion + '\n';

        String signature = signData(stringToSign,privateEncodedKey);

        Walmart.WalmartRequestObject requestBody = new WalmartChatBot.WalmartRequestObject();
        requestBody.channelId = 'c1';
        requestBody.userId = 'u1';
        requestBody.conversationId = 'conv1';
        requestBody.verticalId = 'GROCERY';
        requestBody.surfaceCapability = 'VOICE';

        Walmart.Message messageBot  = new  WalmartChatBot.Message();
        messageBot.typeVar = 'TEXT';
        messageBot.query = 'What is the nearest Walmart store?';

        requestBody.message = messageBot;

        Walmart.Metadata meta =  new WalmartChatBot.Metadata();

        requestBody.metadata = meta;

        Walmart.UserProfile userProfileData = new WalmartChatBot.UserProfile();
        userProfileData.key = 'value';

        requestBody.userProfile = userProfileData;

        Http http = new Http();
        HttpRequest req = new HttpRequest();
        
        req.setEndpoint(baseURL);
        req.setHeader('Content-Type','application/json');   
        req.setHeader('WM_CONSUMER.ID',consumerId);  
        req.setHeader('WM_CONSUMER.INTIMESTAMP',timestamp);  
        req.setHeader('WM_SEC.AUTH_SIGNATURE',signature);  
        req.setHeader('WM_SEC.KEY_VERSION','1');  
        req.setHeader('WM_CONSUMER.ACCOUNT_ID','1111');       
        req.setMethod('POST');
        req.setBody(JSON.serialize(requestBody).replaceAll('typeVar', 'type'));//Replacing the type reserved key here.
        
        httpResponse response = http.send(req);
        System.debug('Service Response: '+response.getBody());

        return message;
    }

    /**
     * Signs the data to generate a WM_SEC.AUTH_SIGNATURE
     * String stringToBeSigned [Ex String stringToSign  consumerId + '\n' + timestamp + '\n' + privateKeyVersion + '\n';]
     * String encodedPrivateKey [You need to convert your .pem private key  to PK8 format using this command: openssl pkcs8 -topk8 -nocrypt -in key.pem -outform PEM
     * you need to paste the text between -----BEGIN PRIVATE KEY----- and -----END PRIVATE KEY-----]
     */
    public static String signData(String stringToBeSigned, String encodedPrivateKey){
     
        Blob encodedKeyBytes    = EncodingUtil.base64Decode(encodedPrivateKey);   
        Blob rs256sig           = Crypto.sign('RSA-SHA256', Blob.valueOf(stringToBeSigned), encodedKeyBytes);
        return EncodingUtil.base64Encode(rs256sig);
    }

    public class WalmartRequestObject{
        String userId;
        String channelId;
        String conversationId;
        String verticalId;
        String surfaceCapability;
        Message message;
        Metadata metadata;
        UserProfile userProfile;
    }

    public class Message{
        String typeVar;
        String query;
    }

    public class Metadata{

    }

    public class UserProfile{
        String key;
    }
}