Logged out user clicks reply and asked to login – How to switch from default WP login to UM login

Hi.  I love Ultimate Member!  Thank you for a great product.  We have just added Forum WP to our staging site and we are trying to figure out how to change this.  Here's the scenario.  

A logged out visitor navigates to our forums, finds a topic and goes to reply to a topic which requires logging in.

For example on our staging site, the user could be on this page (<a href="Test">Topics) and then clicks on 'Reply' which pops up the default WordPress login page. 

But we don't want the default WordPress login to pop up, instead we want the UM login screen we built to appear (e.g.  Login). 

Can you advise on how we can do this?

Thank you.