<apex:page controller="prac1">
<apex:form >

<table cellpadding="2" cellspacing="2">
<!--
    <tr>
        <td style="font-weight:bold;">Document Type<br/>
          <select id="DocTypeOnBase" value="{!searchDocType}">
            <option value=""></option>
            <apex:repeat value="{!DocTypeValues}" var="DocTypeValue">
              <option value="{!DocTypeValue}">{!DocTypeValue}</option>
            </apex:repeat>
          </select>
        </td>
      </tr>
      -->
      <apex:selectList size="1" id="pb1" value="{!selectedValue}" >
       <apex:selectOptions value="{!pickListValues}" />
      </apex:selectList>
    <tr>
      
        <td style="font-weight:bold;">Document Text<br/><br></br>
        <apex:inputtext value="{!searchText}" id="DocumentText" rendered="true" />
        </td>
    </tr>
     <tr>
        <td><br/>
        <apex:commandlink action="{!processLinkClick}" target="_blank" ><b>Search OnBase</b></apex:commandlink>&nbsp;&nbsp;
        </td>      
      </tr>
   </table>
   </apex:form>
</apex:page>

public with sharing class prac1 {

    
    
    public String DocTypeValues { get; set; }
    public string searchText{get;set;}
    public string searchDocType{get;set;}
    
    // new code added 
    public List<selectoption> pickListValues{get;set;}
    public String selectedValue { get; set; }

    public prac1(){
    pickListValues= new List<selectOption>();
    pickListValues.add(new selectoption('val1','Value1'));
    pickListValues.add(new selectoption('val2','Value2'));
    pickListValues.add(new selectoption('val3','Value3'));
    
    }
    
    
    public PageReference processLinkClick() {

      return new PageReference('https://onbasetest.rti.org/AppNet/docpop/docpop.aspclienttype=activex&cqid=196&fts='+selectedValue);
 }
    
  
   
   
}