Display Text Box on Specific Posts only

Hi,

I have added a text box after entry content on my all posts…

I want this text box to be shown only to specific posts that has a outbound(external) link to specific website…

For example if my post has a link to wikipedia. com ,

I want that text box to be show only to the posts that has wikipedia outgoing links in it…

I don’t want other posts to have that text box if it dose have a particular link in it