Had a look a long time for this so sharing here.
To get the tier ID, retrieve a member already assigned to the tier over API or find it in the URL when editing the tier.
PUT to /admin/members/{member_id}
{
"members": [
{
"labels": [
{
"name": "Gift sub"
}
],
"status": "comped",
"tiers": [
{
"id": "123456789",
"expiry_at": "2025-03-07T16:48:01.826Z"
}
]
}
]
}