Steps for get PRIVATE API KEY:
Step 1: Login to your Sendgrid Admin Portal. Sendgrid Portal
Step 2: Go to your Sendgrid account, click on setting and click on API Keys.
Reference image
Step 3: Then click on Create API Keys.
Reference image
Step 4: Give name to API Key and check on Full Access and click on Create & View.
Reference image
Step 5: Copy the API Key
Reference image
Step 6: Paste in below text field
Step 7: Click on Verify button.