public String fName { get;set; }
public String lName { get;set; }
public String email { get;set; }
public String accName { get;set; }
public Id AccId { get;set;}
public List<Account> accList { get;set;}
Public Opportunity currentRecordId { get; set; }

public addApprenticeDetails_CX(ApexPages.StandardController controller){
   currentRecordId = (Opportunity)controller.getRecord();
}
    public PageReference callLookup(){
        accList = [Select id,Name from Account];
        PageReference pr = new
        PageReference('/apex/addApprenticeDetailsAccountLookup');
        pr.setRedirect(false);
        return pr;
    }

    public PageReference goBack(){
    accName = ApexPages.currentPage().getParameters().get('AccName');
    AccId = ApexPages.currentPage().getParameters().get('AccId');
    PageReference pr = new PageReference('/apex/addApprenticeDetails');
    pr.setRedirect(false);
    return pr;
    }

    public PageReference save(){
        if(lName == null || lName == ''){
        ApexPages.addmessage(new
   ApexPages.message(ApexPages.severity.WARNING,
   'Last Name is Required!'));
        return null;
        }
        if(email == null || email == ''){
        ApexPages.addmessage(new
    ApexPages.message(ApexPages.severity.WARNING,
    'Email is Required!'));
        return null;
        }
        else{
            Contact con = new Contact();

                    con.FirstName = fName;
                    con.LastName = lName;
                    con.Email = email;
                    con.AccountId = AccId;
                    con.RecordTypeId =
Schema.SObjectType.Contact.getRecordTypeInfosByName().
get('Corporate Contact').getRecordTypeId();
                    con.Third_Party_Opportunity__c =
ApexPages.currentPage().getParameters().get('Id');
                insert con;


    PageReference pr = new PageReference('/apex/addApprenticeDetails');
    pr.setRedirect(true);
    return pr;

    }
    }
public list<Account> accs { get; set; }
public String searchString { get; set; }
public addApprenticeDetails_CX(){

}
public void search(){
    string searchQuery = 'select name, id from account where name like
'%'+searchString+'%'Limit 20';
    accs = Database.query(searchQuery);

}
public void remove(){
    accs.clear();
}
}

<apex:page standardController="Opportunity"
extensions="addApprenticeDetails_CX">
<apex:form >
<apex:actionFunction name="callAction" action="{!callLookup}"/>
<apex:pageBlock >
<apex:pageBlockSection columns="1">
<apex:pageMessages />
<apex:inputText label="First Name" value="{!fName}"/>
<apex:inputText label="Last Name" value="{!lName}"/>
<apex:inputText label="Email" value="{!email}"/>
<apex:inputText label="Lookup Account" style="float:left;" value="
{!accName}">
<apex:image
value="https://api.icons8.com/download/
19343120d27c16dd3e9d21ad3aa637f94fd4d5fa/Android_L/PNG/256/
Very_Basic/search-256.png" height="20" onclick="callAction()"/>
</apex:inputText>
</apex:pageBlockSection>
<center>
<apex:commandButton value="Save" action="{!save}"/>
</center>
</apex:pageBlock>
</apex:form>
</apex:page>

<apex:page standardController="Opportunity"
extensions="addApprenticeDetails_CX" tabStyle="Contact">
<apex:form >
<apex:inputText value="{!searchString}" label="Input"/>
<apex:commandButton value="Search Accounts" action="{!search}"/>
<apex:commandButton value="Clear Accounts" action="{!remove}"/>
<apex:pageBlock title="Search Results">
<apex:pageBlockSection columns="1">
<apex:repeat value="{!accs}" var="acc">
<apex:commandLink value="{!acc.Name}" action="{!goBack}">
<apex:param value="{!acc.Name}" name="AccName"/>
<apex:param value="{!acc.Id}" name="AccId"/>
</apex:commandLink>
</apex:repeat>
</apex:pageBlockSection>
</apex:pageBlock>
</apex:form>
</apex:page>