Hi @sudeepg4,
You can send an email campaign with the "send now" option that we use in the UI; however you do still have to select a contact list to send to. The email campaign cannot be sent to just a single email address.
In order to replicate the "send now" option you will use the schedule endpoint and send an empty JSON body. You did not specify which version of the API you are using, so here is an example from our v3 API.
Post /emails/activities/{campaign_activity_id}/schedules
{ }
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