Generating URL dynamically pre-populate custom fields values while creating new records

Generating URL to pre-populate Contact fields

 /*Now consider a scenario where we want to pre-populate below contact fields:
  LastName
  Email
  Birthdate
  Level__c (custom field)
  Category__c(custom lookup field)  
  */
  
  string redirectUrl='';
	String baseURL = Url.getSalesforceBaseUrl().toExternalForm();
	String objectKeyPrefix = Schema.getGlobalDescribe().get('Contact').getDescribe().getKeyPrefix();
	string bdate = EncodingUtil.urlEncode('25/05/1988', 'UTF-8' );
	string lname = EncodingUtil.urlEncode('Sunil Kumar', 'UTF-8' );
	string emailFieldvalue = EncodingUtil.urlEncode('[email protected]', 'UTF-8' );
	Id CategoryFieldId = 'a079000000lA1aP'; 
	string categoryFieldValue = 'test cat'; 
	string TypeCustomFieldValue = EncodingUtil.urlEncode('New', 'UTF-8' );
	//populating birthdate and last name
	redirectUrl = baseURL + '/' + objectKeyPrefix + '/e?con7=' + bdate + '&name_lastcon2=' + lname ; 
	//populating email value
	redirectUrl = redirectUrl + '&con15=' +  emailFieldvalue;
	//finding custom field ids
	Map<string,string> fieldToIdmap= ToolingAPIUtility.findCustomFieldsid('Contact');
	if(fieldToIdmap.get('Type__c') != null){
		redirectUrl= redirectUrl + '&'+ fieldToIdmap.get('Type__c') + '=' + TypeCustomFieldValue;
    }
	//for custom lookup fieldId
	if(fieldToIdmap.get('Category__c') != null){
		redirectUrl= redirectUrl+ '&CF'+ fieldToIdmap.get('Category__c') + '=' + categoryFieldValue;
		redirectUrl= redirectUrl + '&CF'+ fieldToIdmap.get('Category__c') + '_lkid=' + CategoryFieldId;
    }
	redirectUrl = redirectUrl + '&retURL=%2F'+ objectKeyPrefix + '%2Fo';
	system.debug('*****redirectUrl:'+redirectUrl);

ToolingAPIUtility.cls

public class ToolingAPIUtility{

    public static Map<string,string> findCustomFieldsid(string ObjName){
        Map<string,string> FieldToFieldidMap = new Map<string,string>();
        string customObjectId;
        string TableEnumOrIdValue;
        String baseURL = Url.getSalesforceBaseUrl().toExternalForm();
        if(ObjName.endswith('__c')){
            ObjName = ObjName.substring(0,Objname.length()-3);
            system.debug('*******Objname:'+Objname);
            String query = 'SELECT Id From CustomObject Where DeveloperName = \'' + ObjName+ '\'';
            string endpoint = baseURL + '/services/data/v38.0/tooling/query/?q=';
            endpoint += EncodingUtil.urlEncode(query, 'UTF-8');
            Map<string, string> httpHeaders=new Map<string, string>();
            httpHeaders.put('Authorization', 'Bearer ' + UserInfo.getSessionId());
            httpHeaders.put('Content-Type', 'application/json');
            HttpRequest request = createHTTPRequest(endpoint, 'GET', httpHeaders);
            HttpResponse response = sendHTTPRequest(request);
            
            system.debug('*****object**response:'+response);
            customObjectId = response.getBody().substringAfter('"Id":"').substringBefore('"');
            system.debug('*****object**customObjectId :'+customObjectId );
            TableEnumOrIdValue = customObjectId ;
        }else{
            TableEnumOrIdValue = ObjName;
        }
        if(TableEnumOrIdValue !=null && TableEnumOrIdValue !=''){
            String query = 'SELECT Id, DeveloperName From CustomField Where TableEnumOrId = \'' + TableEnumOrIdValue+ '\'';
            string endpoint = baseURL + '/services/data/v38.0/tooling/query/?q=';
            endpoint += EncodingUtil.urlEncode(query, 'UTF-8');
            Map<string, string> httpHeaders=new Map<string, string>();
            httpHeaders.put('Authorization', 'Bearer ' + UserInfo.getSessionId());
            httpHeaders.put('Content-Type', 'application/json');
            HttpRequest request = createHTTPRequest(endpoint, 'GET', httpHeaders);
            HttpResponse res = sendHTTPRequest(request);
            String response=res.getBody(); 
            system.debug('*******response:'+response); 
            SYSTEM.JSONParser parser = SYSTEM.JSON.createParser(response);
            while (parser.nextToken() != null) {
                string fieldName='';
                string fieldId='';
                if ((parser.getCurrentToken() == JSONToken.FIELD_NAME)){
                    //system.debug('*********parser.getText():'+parser.getText());
                    if(parser.getText()=='Id'){
                        parser.nextToken();
                        fieldId= parser.getText();
                        system.debug('*************fieldId:'+fieldId);
                        parser.nextToken();   
                    }if(parser.getText()=='DeveloperName'){
                        parser.nextToken();
                        fieldName=parser.getText();
                    }
                    if(fieldName!='' && fieldId !=''){
                        FieldToFieldidMap.put(fieldName+'__c',fieldId.substring(0,15));
                    }
                }
            }
            system.debug('*********FieldToFieldidMap:'+FieldToFieldidMap);
        }
        return FieldToFieldidMap;
    }
    
    public static HTTPRequest createHTTPRequest(string endPointURL, string HTTPMethod, Map<string,string> HTTPHeaders){
        HttpRequest req = new HttpRequest();
        req.setEndpoint(endPointURL);
        req.setMethod(HTTPMethod);
        for(string ss : HTTPHeaders.keyset()){
            req.setHeader(ss, HTTPHeaders.get(ss));
        }
        return req;
    }
    public static HTTPResponse sendHTTPRequest(HTTPRequest req){
        HttpResponse res;
        if(!SYSTEM.Test.isRunningTest()){
            Http h = new Http();
            res = h.send(req);
        }
        return res;
    }   
}