No Refresh Token created when generating Access Token

The issue is the same as discussed in https://help.zoho.com/portal/en/community/topic/missing-refresh-token-in-respose - no matter what parameter I send, the refresh token is always missing.

I have tried all combinations of access_type=offline and prompt=consent, and the refresh token is NEVER included. These are the parameter I send:

client_id=<removed>

redirect_uri=<removed>

So it seems to me that there is more going on than what https://www.zoho.com/accounts/protocol/oauth/web-apps/access-token.html explains. What am I missing?

In case it matters, I'm following https://www.zoho.com/campaigns/help/developers/access-token.html, and am using a client for server-based applications.

Thanks!