Affiliate Magnet will not allow you to customize the text that is attached before the message. Up till now, whenever the bot replaced any affiliate link, "From @member" was added to the beginning. Now, you will be able to customize that text.
You can set your own header using the slash command /set header <your header text>.
Special characters {m} and {@m}
{m} and {@m} stand for member and mention member respectively. Whenever
the bot replaces a link, it will attach the header with the member's username
or nickname in place of {m} and a mention (@) in place of {@m}.
You can see examples here.