gistfile1.txt

terraform {
  required_version = "~> 1.0.0"

  backend "s3" {
    bucket         = "<bucket>"
    key            = "admin-global/terraform.tfstate"
    dynamodb_table = "terraform-state-lock"
    region         = "eu-west-1"
    encrypt        = "true"
  }
}

gistfile2.txt

{
    "UserId": "<uid>:<email>",
    "Account": "<acc_nr>",
    "Arn": "arn:aws:sts::<acc nr>:assumed-role/AWSReservedSSO_AdministratorAccess_<random_string>/<email>"
}

gistfile3.txt

backend "s3" {
  ...
  role_arn       = "arn:aws:iam::<acc nr>:role/atlantis"
}

gistfile4.txt

provider "aws" {
  assume_role {
    role_arn = "arn:aws:iam::<acc nr>:role/atlantis"
  }
}