This tutorial will teach you how to get a custom spawn egg. Ever since 1.11 came out there was no need to make spawn eggs of non boss mobs that don't have a spawn egg. But what if you made a mob but you wan't to make a spawn egg of that mob than keep reading. I will show you how to make a wither spawn egg but you don't haft to make a wither spawn egg.
Step 1.

Place a command block down and type this command /give @p minecraft:spawn_egg 1 0 {Display:{Name:"SpawnWither"},EntityTag:{id:"enderman"}}
To get a command block type in the chat /give @p minecraft:command_block 1
Step 2.

Get a repeat command block and a chain command block. Type in the repeat command block /execute @e[type=enderman,name=SpawnWither] ~ ~ ~ /summon wither ~ ~ ~
And then place a chain command block next to the repeat command block. Make sure the arrows are facing the same direction. And type in this command /kill @e[type=enderman,name=SpawnWither]
Put the chain command block on always active. To get a chain command block type in this command in the chat /give @p minecraft:chain_command_block 1
To get a repeat command block type in this command in the chat /give @p minecraft:repeating_command_block 1
Step 3.

Activate the repeat command block with redstone and then use your spawn egg the wither should spawn. Have fun!

Note:If you do not have the kill command activated it will contently spawn withers.