Steps for get PRIVATE API KEY:

Step 1: Login to your Omnisend Admin Portal. Omnisend Portal

Step 2: Go to your Omnisend account, click on your username and then click on Store setting.
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Step 3: Then click on API Keys
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Step 4: Click on Create API Key to generate a new API Key.
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Step 5: Give name to API Key ,check all permissions and click on Create API Key button.
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Step 6: Copy the Private API Key.
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Step 7: Paste in below text field.

Step 8: Click on Verify button.