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, in order to add a dedication, mark with a V the option "Write a dedication/comment"
After you have checked the option to add a dedication, a window will open where you can write the note/dedication you wanted:
After completing the donation process, the dedication will be displayed under the donation details in the "Recent Donations" section of the organization's campaign page.