at the bottom of the page there are some links to external social media websites
the urls have two parameters: page and site ... changing the site parameter to something else other than predefined words ("facebook", "twitter", "instagram") will yield the flag:
for example given the url http://192.168.99.100/index.php?page=redirect&site=instagram1
the page produces the flag:
GOOD JOB HERE IS THE FLAG : B9E775A0291FED784A2D9680FCFAD7EDD6B8CDF87648DA647AAF4BBA288BCAB3