If you're editing the back-half of a link and see the error This custom Microsite URL is taken, you'll need to enter a different back-half.
This is because someone has already used that back-half with that name.
Since s.id links are case sensitive, try changing the capitalization. You can also add other characters, like numbers or a hyphen.
For example, if someone has used s.id/example, and you wanted to use the sample back-half, you could try s.id/example1 or s.id/example2022.