use whatever tool you like to call the API, postman is certainly OK.

Just follow the official API guide: https://docs.databricks.com/dev-tools/api/latest/gitcredentials.html#operation/create-git-credential

only 3 fields in the payload, and the official example is already for github pat.

N.B. call the API with the SPN access token for API authentication. hereunder a snippet to get the access token

curl -X POST \
"https://login.microsoftonline.com/$tenantId/oauth2/v2.0/token" \
-H "Content-Type: application/x-www-form-urlencoded" \
-d "client_id=$client_id" \
-d 'grant_type=client_credentials' \
-d 'scope=2ff814a6-3304-4ab8-85cb-cd0e6f879c1d%2F.default' \
-d "client_secret=$client_secret" \
| jq -r .access_token