Advertising is a form of earned revenue and fiscally sponsored projects are not allowed to accept payments for earned revenue through their fund.
However, advertising is not the same thing as corporate sponsorship.
For example, acknowledgments, such as a simple “Thanks to our sponsors!” and a logo placement are considered sponsorship. Corporate sponsorship payments are generally tax deductible when contributed to a 501(c)(3) because the sponsorship is generally assumed to not have a fair market value. We can accept contributions that represent corporate sponsorship for the purposes of your project.
Qualitative or comparative descriptions of the sponsor’s goods or services (basically, encouraging someone to buy the product or service, such as “Go to Sponsor, the best in the business!”) is considered advertising. The person or entity purchasing the ad space also maintains control over the content and placement of the ad itself. A good rule of thumb is that, if the purchaser provides you with text to include in your materials, then this is likely advertising.
Advertising income is generally taxable, even when it is received by a 501(c)(3) charity. It is typically considered earned income because the entity purchasing the ad space is getting something in exchange for their money. We cannot accept payments for advertising; you need to handle that yourself outside of your sponsored fund.
You can find more information about this here.