You can set the content of a campaign by setting it's HTML attribute.
$campaign->html = $yourHtml;
In that HTML you can use these placeholders which will be replaced when sending out the campaign:
::unsubscribeUrl::: this string will be replaced with the URL that, when hit, will immediately unsubscribe the person that clicked it
::unsubscribeTag::your tag::: this string will be replaced with the URL that, when hit, will remove the "your tag" tag from the subscriber that clicked it
::webviewUrl: this string will be replaced with the URL that will display the content of your campaign. Learn more about this in the docs on webviews.
If there is no way for a subscriber to unsubscribe, it will result in a lot of frustration on the part of the subscriber. We always recommend using ::unsubscribeUrl:: in the HTML of each campaign you send.