Go to grid view of the entry you want to link, click the "···" icon in the upper right corner
Tap "More" - "Copy As" - "Copy Markdown Link". Now you have copied its link.
Tap into the grid where you want to add this link. And then tap the blank space and tap "Paste" to add the copied link.
Now if you preview this entry, you can tap the link and go to this linked diary.