Sometimes Facebook’s @tag doesn’t work when you’re trying to give photo credit, or other credits, on an upcoming post. If this happens, I’ve found a trick that works.
First, I make the post with the Photo Credit: @companyname as plain text, not a link. Then, I edit the post and/or edit each photo with unlinked photo credit. From edit mode, I put my cursor at the very end of the credit name (last letter). Note the @ sign is already there. This method almost always gives me the dropdown to select the company or individual’s name I’m trying to credit.
I hope this trick helps solve your tagging/crediting woes on Facebook, too!