A/B test for proportions calculating sample size

I want to conduct an A/B test where main metric is ‘logged in visitors/total visitors’ to my website
I assume, and want to verify with you that I’m correct:
- my null hypothesis is that there’s no difference between the 2 groups proportions
- Therefore, it is a chi squared test
If so, how do I calculate the sample size needed for the test? The online calculators talk about CTR, but that’s not a CTR

I would appreciate some theoretical explanation to shade light on this
Thank you!