In order to send an email campaign on behalf of another user, you first need to have that user create the campaign. In the New Campaign screen, you can see who the campaign will send as:

You can also verify who the campaign will send as by clicking on the Edit button:

After the campaign is created, you are free to add steps and prospects. When you click Preview and Send a Test Emails:

You'll see that the email is sent by the user who created the campaign:

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