Of course you can!
After you have chosen the potential campaign you want to donate to, start the donation as usual and click on the "Donate" button
In the donation window, you will be asked to enter the amount of the donation, to add a comment, tick the option "Write a comment"
A text box opens, where you can write your note or dedication. For a formal dedication in honor or in memory of someone, turn on the "Dedicate This Donation" toggle below it.
After completing the donation process, the comment will be displayed under the donation details in the "Recent Donations" section of the organization's campaign page.