
The SSM Parameter Store contains the following JSON:
{
  "username": "admin",
  "password": "password"
}
It is necessary to extract the login and password, and use their values in the Terraform code. To do this, you can use the following construction:
# Should be there before the apply
data "aws_ssm_parameter" "rds-admin-user" {
  name  = "/ARTEM-SERVICES/PROD/RDS/CREDENTIALS"
}
locals {
  additional_rds_username      = jsondecode(data.aws_ssm_parameter.rds-admin-user.value)["username"]
  additional_rds_user_password = jsondecode(data.aws_ssm_parameter.rds-admin-user.value)["password"]
}
And use variables:
local.additional_rds_username local.additional_rds_user_password