I'm on Myrbetriq, and I get botox injections in my bladder every four months or so.
The Myrbetriq helps reduce spasms as I get to the end of my botox cycle. I've never tried to quantify how much it helps, but I seem to have fewer spasms, and therefore less pain, when I'm on it.
Botox helps tremendously with reducing the constant pain and feeling of needing to pee. During the last few weeks before my next botox injections, I'll have increasing bladder pain and spasms, often enough to cause leakage. Those calm down a week or so after botox, and I'm good for a few more months.