Example Search Before Create Apex Controller. This is a sample implementation which interacts with a Transaction Hub which have 'search' and 'publish' REST APIs which return the results in JSON format.

searchAccounts.cls

public without sharing class searchAccounts {

    public String name { get; set; }
    public String op { get; set; }
    public String mdId { get; set; }
    public String accId {get; set;}
    public Account acc {get; set;}
    
    public List<Acc> accs { get; set; }
    
    // Hub Account Object
    public class Acc {
        public String name {get; set;}
        public String Id {get; set;}
        public String MDId {get; set;}        
        public String BillingCity {get; set;}
        public String BillingStreet {get; set;}
        public String AccountNumber {get; set;}
    }

    public searchAccounts(ApexPages.StandardController controller) {
        acc = (Account)controller.getRecord();
        accId = acc.Id;
        name=Apexpages.currentPage().getParameters().get('acc2');
        if(accId==null) {
            accId=Apexpages.currentPage().getParameters().get('acc1');
        }
        if(name==null && accId!=null) {
            name = [select name from account where id=:accId][0].name;
        }
    }
        
    public searchAccounts() {
        accs = new List<Acc>();
    }
    
    //##############################    
    // WS Callout to Search Account in MDM System
    //##############################   
    public List<Acc> getAccounts() {
        if (name==null) return null;
        accs = new List<Acc>();
        Http httpProtocol = new Http();
        HttpRequest request = new HttpRequest();
        request.setEndPoint(System.Label.MDM_URL + '/api/find?name='+EncodingUtil.urlEncode(name,'UTF-8'));
        request.setMethod('GET');
        // Make callout
        HttpResponse response = httpProtocol.send(request);
        try {            
            if(response!=null && response.getBody()!=null) {
                // Parse JSON Results to a List of objects
                List<Object> m = (List<Object>)JSON.deserializeUntyped(response.getBody());
                for(Object k:m) {
                    Map<String,Object> o = (Map<String,Object>)k;            
                    Acc acc = new Acc();
                    acc.Id=(String)o.get('Id');   
                    acc.MdId=(String)o.get('_id');          
                    acc.name=(String)o.get('Name'); 
                    acc.accountNumber=(String)o.get('AccountNumber');
                    acc.billingStreet=(String)o.get('BillingStreet');
                    acc.billingCity=(String)o.get('BillingCity');
                    accs.add(acc);                  
                }
            }
        } catch (Exception e) {
                system.debug(e);
        }
        return accs;                     
    }    
    //#################################
    
    //##############################    
    // WS Callout to Publish an Account from the MDM System
    //############################## 
    public PageReference addAccount() {
        Http httpProtocol = new Http();
        HttpRequest request = new HttpRequest();
        String endpoint = System.Label.MDM_URL + '/publish/'+Apexpages.currentPage().getParameters().get('mdId');
        request.setEndPoint(endpoint);
        request.setMethod('GET');
        HttpResponse response = httpProtocol.send(request);
        return null;                     
    }
    //#################################
           
}