A/B testing: How to calculate p-value on post test segments?

My question on A/B testing is about doing post test segmentation analysis.

For example:

I run an A/B test on my website to track bounce rate. On the
treatment group, i put a video to explain my company. On the control
group i put just plain text. I pick a segment of users who are first
time users from USA to be split 50/50 into the 2 groups.

Metric that i am tracking is average bounce rate (assume 20%). 
Power effect (0.8)
effect size i expect to see(10% so bounce rate should fall to (20% - 0.10 * 20% = 18%))
Calculated sample size required is say 1000 for each group. 

Say i run the test for the correct amount of time. At the end of the test, i get a p-value of 0.06. i do not reject the null hypothesis.

However, when i do post test segmentation analysis, for example, i saw
that users who signed up for a free trial, 44% of them played the
video.

In this case, how do i calculate if the 44% was significant? (while taking into account the multiple comparison problem?)
Like in the Airbnb experiment, they did post segmentation analysis on the browser type and was able to calculate the p-value.

My approach

Does this mean that for every segment that i want to analyze, i need to have at least 1000 samples? Also how would i recalculate the p-value given that the p-value of this A/B test was already generated above as 0.06?