Please do your research before posting, I found an easily working skript:

Code (Text):

1.  
2. on join:
3.     create bossbar titled "&3&lWELCOME, &b%player%" and id "welcome.%player's uuid%" for player with progress 100 with colors aqua
4.     wait 5 ticks
5.     loop 100 times:
6.         set {_Value} to value of bossbar "welcome.%player's uuid%"
7.         remove 1 from {_Value}
8.         set bossbar "welcome.%player's uuid%" value to {_Value}
9.         wait 3 ticks
10.     remove bossbar "welcome.%player's uuid%"
11.  

This requires skRayFall, a Skript addon.