target of hyperlink in app web changes automatically

We are using some apps in our Intranet. Within an app we want to have hyperlinks in a TextBox linking to other Intranet pages. After having inserted and saved the target url is transformed like this:

http://app0123/SitePages/Homepage.aspx

Originally we wanted to link to

http://intranet/SitePages/Homepage.aspx

It seems that the host url is transformed relatively to the app address.

How can we get around this?