High Heat Mortar Primer
The best solution is to use RefMix, or another pre-mixed refractory mortar. Forno Bravo stock RefMix in California, and we highly recommend it. It is pre-mixed (just add water), sets hard, is easy to work with, fully cured quickly, and is heat resistant to 1700F. It's made specifically for pizza ovens and fireplaces, and there is even a pizza oven graphic on the bag.
If you don't want to worry about shipping RefMix, or want to save money, you can make your own mortar, using either fireclay and Portland cement or fireclay and Calcium Aluminate. Fireclay is a heat resistant clay made up of aluminate and silica. When you mix fireclay with Portland cement, sand and lime, you create a product that is more heat resistant than basic mason's mortar (Portland cement with sand and lime).
Measure your ingredients by volume (use a bucket or shovel to measure), and mix only the amount you will us within an hour or so.
1 part Portland cement
Still, fireclay mortar is less heat resilient and thermally conductive than a true refractory mortar. If you are building a commercial oven, or simply want to "go the extra yard," but do not want to use RefMix, you can make your own refractory mortar with Calcium Aluminate.
First, a word of caution. Working with calcium aluminate can be challenging. If you get the mix, or water wrong, it won't set correctly. It partially sets very quickly, and you cannot re-hydrate it, so you have to mix it and use it in small batches. Still, if you are trying to save money and want or need the heat resilience, heat conductivity and longevity of a true aluminate mortar, it works.
1 part Calcium Aluminate