How to spawn King Slime?

I play Terraria on iOS. I defeated the Eye of Cthulhu, Skeletron and Eater of Worlds. I knew that I had to face to Wall of Flesh next. But then while I was casually going through the Wiki, I came across King Slime, which randomly spawns during the Slime Rain event or can be summoned by the Slime Crown. But I'm neither able to craft a crown in the demon altar, nor able to witness the Slime Rain event.

According to the Terraria Recipes wikia page,

Crafting is done through the crafting bar, visible on the left side of the interface below the player's Inventory, which presents all craftable items available to the player. Items will only appear in this list if the player has the ingredients in their inventory.

That being said, the number of ingredients to make the Slime Crown differ between console/mobile and the desktop version.

In the desktop version, you will need:

• 1 Gold or Platinum Crown
• 20 Gel

However, in the console/mobile version, you will need

• 1 Gold or Platinum Crown
• 99 Gel

Unless you have a Gold or Platinum Crown and 99 Gel in your inventory, the option to craft the Slime Crown will not appear.