<aura:component>
     <lightning:button label="Create" onclick="{!c.createElements}" variant="brand"/> <br/>
     <div aura:id="createdynamicInputText"></div>
     <lightning:button label="Set Value" onclick="{!c.setValue}" variant="brand"/> <br/>
</aura:component>

createElements : function(component, event, helper){
         $A.createComponent(
                "lightning:input",
                {
                    "aura:id":"objectField",
                    "label": "User Name",
                    "type": "text"
                },
                function(msgBox){
                    if (component.isValid()) {
                        var targetCmp = component.find('createdynamicInputText');
                        var body = targetCmp.get("v.body");
                        body.push(msgBox);
                        targetCmp.set("v.body", body);
                    }
               }
           );
    },
    setValue : function(component, event, helper){
        component.find("objectField").set("v.value","Testing");
    }