<aura:component implements="forceCommunity:availableForAllPageTypes>
    <ui:button label="CustomButton" press="{!c.createRecord}"/>
</aura:component>

({
    createRecord : function(component, event, helper) {
        var createRecordEvent = $A.get("e.force:createRecord");
        createRecordEvent.setParams({
            "entityApiName" : "CustomObjectAPI__c"
        });
        createRecordEvent.fire();
    }
})

.THIS{
 YOUR CSS

}