post https://api.mixpanel.com/track#create-identity
You can also use the import endpoint: https://api.mixpanel.com/import/
Identify Criteria:

Required Event Object attributes
Event Object property | Type | Description |
---|---|---|
event | String required | value must be: $identify |
properties | Object required | |
properties.distinct_id | String optional | The distinct ID post-identification (same as $identified_id - it will be inferred from $identified_id if not included) |
properties.$identified_id | String required | A distinct_id to merge with the $anon_id. |
properties.$anon_id | String required | A distinct_id to merge with the $identified_id. The $anon_id must be UUID v4 format and not already merged to an $identified_id. |
properties.token | String required | The project token. |