This likely won't work. Money is fungible, so unless the total donations so earmarked exceeds the planned SI funding for that cause, they won't have to change anything. They're under no obligation to not defund your favorite cause by exactly the amount you donated, thus laundering your donation into the general fund. (Unless I misunderstand the relevant laws?)

EDIT NOTE: The post used to say vast majority; this was changed, but is referenced below.