How to maintain muscle?

I'm on my 3rd week of Zep and love the weight-loss but I am rapidly also losing muscle mass. I've added a 25g protein shake in the morning this week. for dinner I am generally having lean meat (chicken or turkey) or salmon with veggies.

I'm guessing the biggest issue is just a lack of calories to maintain muscle mass, am I right? How have you all combatted this issue? Do you force yourself to eat protein even when you're not hungry?

Or alternatively should I focus on arriving at my goal weight and then worry about putting muscle back on?

On an unrelated note, I lurked here for a couple of months and was so impressed by many of your weight loss journey and that's what made me give it a shot too (pun intended)!