Sample Code:

Component:

<aura:component implements="force:appHostable" >  
      
    <aura:attribute name="strText" type="String"/>  
      
   <div class="slds-box slds-theme_default">  
  
       <span onkeypress="{!c.callKeyUp}">  
             
           <lightning:input label="Enter Text"  
                            value="{!v.strText}" />  
             
       </span>  
          
    </div>  
      
</aura:component>  

Component Controller:

({  
  
    callKeyUp : function(component, event, helper) {  
          
        if ( event.keyCode == 13 )  
            alert(component.get("v.strText"));  
          
    }  
      
})  

Output:

Enter some text and click Enter button.