From The P Systems Webpage

Get the name of the page that you want to link to, from the header line. Copy the name.

Go to the page where the link should be. Click the underlined Ab. Paste in the copied name. Make an | sign. After that comes fill in what you want the link to say.

For example:

"[Psystems:NameofthePage|Link Text]"

(Note: it will be in cornered brackets.)