Hi 

Try this code 

<apex:page controller="RedirectTOStandardPageController">
 <apex:form>
     <apex:pageBlock>
         <apex:selectList value="{!selectedAccountId}"  multiselect="false" size="1">
             <apex:selectOptions value="{!selectedList}"/>
             
         </apex:selectList>
         <apex:commandButton value="Go" action="{!redirect}"/>
     </apex:pageBlock>
 </apex:form> 
</apex:page>

 My Controller is 

public class RedirectTOStandardPageController{
    public List<Account> accountList;
    public List<SelectOption> selectedList;
    public Id selectedAccountId{get;set;}
    public RedirectTOStandardPageController(){
        accountList = new List<Account>();
        accountList = [SELECT id, Name FROM Account];
        
    }
    public List<Account> getAccountList(){
        return accountList;
    }
    
    public List<SelectOption> getSelectedList(){
        selectedList = new List<SelectOption>();
        for(Account act:accountList){
          selectedList.add(new SelectOption(act.id,act.Name)); 
        }
        return selectedList;
    }
    
    public PageReference redirect(){
        System.debug('@@@@@@@@@@@@@@@@ACCOUTID@@@@@@@@@@'+selectedAccountId);
        PageReference samePage= new PageReference('https://ap1.salesforce.com/'+selectedAccountId);
        samePage.setRedirect(true);
        return samePage;
    }
}        

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