Custom Welcome Message For Vanilla,Bukkit,Spigot

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             Welcome Message
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     CHAT
         FIRSTLY DO
/scoreboard objectives add FirstTimer dummy
THEN IN A REPEATING CMD BLOCK ALLWAYS ACTIVE PUT
scoreboard players add @a FirstTimer 0
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       SECONDLY DO
    CHAIN COMMAND BLOCK ALLWAYS ACTIVE
/execute @a[score_FirstTimer=0] ~ ~ ~ [MAKE A TELLRAW USING minecraftjson.com and TO ADD THERE USERNAME DO
,{"selector":"@p","color":"dark_aqua"} WHERE EVER U WANT IT TO BE]

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        THIRDLY DO
      CHAIN COMMAND BLOCK ALLWAYS ACTIVE
 /execute @a[score_FirstTimer=0] ~ ~ ~ summon fireworks_rocket ~ ~ ~ {LifeTime:1,FireworksItem:{id:fireworks,Count:1,tag:{Fireworks:{Explosions:[{Type:0,Flicker:1,Trail:0,Colors:[1703927],FadeColors:[16777215]}]}}}}

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         FOURTHLY DO
 CHAIN COMMAND BLOCK ALLWAYS ACTIVE
scoreboard players set @a FirstTimer 1
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