Ignore the times below as they're just to aid with my query...

I'm looking to book an advance train ticket for a specific time.

So, if I'm catching a tube from a London station with just one change at Euston where I catch a train to my final destination, is the time on that advance ticket from the tube station or from Euston?

If it's the former, does that then tie me into a specific time for my train at Euston as well?

Thanks in advance for any clarification on this.