<aura:application extends="force:slds">
    <form aura:id="form" target="iframe" action="#" onkeypress="{!c.blockEnter}">
        <lightning:input type="text" label="Name" name="name" />
        <lightning:button label="Add to Autocomplete" iconName="utility:save" onclick="{!c.saveAutoComplete}" />
    </form>
    <iframe id="iframe" name="iframe" class="slds-hidden" />
</aura:application>

({
    blockEnter: function(component, event, helper) {
        if(!component.get("v.allowed")) {
            if(event.which === 13) {
                event.preventDefault();
            }
        }
    },
    saveAutoComplete: function(component, event, helper) {
        component.set("v.allowed", true);
        component.find("form").getElement().submit();
        component.set("v.allowed", false);
    }
})