Terraform require only specific values in object

Consider the following Example:

variable "myvar" {
  type = object({
    key1 = string
    key2 = string
    key3 = string

Is it possible to define an object which requires the structure defined above, but additionally allows to specify arbitrary keys for input? I don’t want to use optional() for the values here, since the additional keys are not known. All specified keys should then be applied using merge(var.myvar) on a resource.