To make a Webservice callout in batch Apex, we have to implement Database.AllowsCallouts interface in the class definition. Here is an example to make webservice callout in batch apex.

Batch Apex:

global class AccountBatchApex implements Database.Batchable<sObject>, Database.AllowsCallouts{
    
    global Database.QueryLocator start(Database.BatchableContext bc){
        String soqlQuery = 'SELECT Name, AccountNumber, Type From Account';
        return Database.getQueryLocator(soqlQuery);
    }
    
    global void execute(Database.BatchableContext bc, List<Account> scope){
        
        for (Account acc : scope){
            if(acc.Type.equals('Customer - Direct')){
                try{
                    HttpRequest request = new HttpRequest();
                    HttpResponse response = new HttpResponse();
                    Http http = new Http();
                    
                    String username = 'YourUsername';
                    String password = 'YourPassword';
                    Blob headerValue = Blob.valueOf(username + ':' + password);
                    String authorizationHeader = 'BASIC ' + EncodingUtil.base64Encode(headerValue);
                    
                    request.setHeader('Authorization', authorizationHeader);
                    request.setHeader('Content-Type', 'application/json');
                    request.setEndpoint('Your Endpoint URL');
                    request.setMethod('POST');
                    request.setBody('Information to Send');
                    response = http.send(request);
                    if (response.getStatusCode() == 200) {
                        String jsonResponse = response.getBody();
                        System.debug('Response-' + jsonResponse);
                    }
                }
                catch(Exception){
                    System.debug('Error-' + e.getMessage());   
                }
            }
        }
    }
    
    global void finish(Database.BatchableContext bc){
        
    }
}

Note: Total number of callouts (HTTP requests or Web services calls) is 100, that means if you have one callout in your execute method you can keep batch size as 100.