curl 'https://api.paypal.com/v1/oauth2/token' \
-H "Content-Type: application/x-www-form-urlencoded" \
-H "Authorization: Basic QWZVa...==" \
-d 'grant_type=refresh_token&refresh_token=MFYQ...'
What is
Authorization: Basic QWZVa...== ??
and retur true;