Please consider following improvements of Link Manager:
a) Make "Portal Page" selector default, instead of URL
b) When entering Link Manager and Url IsNot empty and Url startswith "\" then try to search page in portal pages and make "Portal Page" selector default.
a) Make "Portal Page" selector default, instead of URL
b) When entering Link Manager and Url IsNot empty and Url startswith "\" then try to search page in portal pages and make "Portal Page" selector default.