Unless exempted, most independent expenditures must include the following disclaimer, as required by the FEC:
Paid for by [insert individual or group name] [insert individual or group physical address or web address]. Not authorized by any candidate or candidate’s committee.
Example: Paid for by Indivisible Project (indivisible.org). Not authorized by any candidate or candidate’s committee.
The disclaimer must also meet the following formatting requirements:
1. Must be clearly readable – ideally 12-point font with a reasonable degree of color contrast (e.g., black text on a white background, or the same degree of contrast as the color of the largest text and the background color); and
2. Must be set apart in a separate “bounding” box and the frame of the box should be the same color of the disclaimer text so it stands out.
For more info and examples, check out this FEC brochure. The brochure is pretty old but the info is still valid.