Hi AichaSF,

You can try this like:

COntroller:
public with sharing class add
{
    public String res{get;set;}

    public PageReference doAdd() {
        <Object Name> obj = [Select Id, Name From <Object Name> Where Id =: res];
        if(obj == null)
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.FATAL, 'Record not found.'));
        
        return null;
    }
}
/*****************/
Page:
<apex:page controller="AddController" id="thePage">
    <apex:form id="theForm" >
        <apex:pageMessages id="someMessages" />
        <apex:pageBlock id="theBlock" >
            <apex:inputText id="test" value=""/>
            <input type="button" value="Click" onclick="doAd()"/> <!-- Your button here or YOu can try below line-->
            <!-- <apex:commandButton value="Click" onClick="doAd()" />-->
        </apex:pageBlock>

        <apex:outputPanel id="out">
            <apex:outputText value="{!res}"/>
        </apex:outputPanel>
        <apex:actionFunction name="doAdd" action="{!doAdd}" reRender="out" >
            <apex:param name="firstParam" assignTo="{!res}" value="" />
        </apex:actionFunction>
    </apex:form>
    
    <script type="text/javascript">
      
        function doAd(){
        var recId = document.getElementById('thePage:theForm:theBlock:test').value ;
        if(recId) {
            doAdd(recId);
        }
        else {
            alert('Plz! provide value for this field');
        }
    }
    </script>
</apex:page>
/****************************************/
Might be it will help you !!
Thanks
Niraj