To create a custom world, press “Singleplayer”.
As shown above, click the “Create New World” button at the bottom.
To generate the world with a custom seed, press “More World Options” and an additional menu will appear. Here you can add your custom seed, change the world type, or choose to generate structures.
In the text box at the top of the screen, add in whatever seed you want. Since version 1.18, Mojang has added the seed parity feature. This allows it that all seeds can be shared between Bedrock and Java editions.
You can enter letters and numbers into the text box, as shown above!
At the bottom of the screen, press “Create New World” and your new world will generate.