HELP - Specific Backlink Question

assume the following -
1 - the main domain is a PR5 - example - maindomain.com - PR5
2 - and the profile page that I have my link on has no PR - example - forum.maindomain.com/profile/7843667289 - PR n/a

QUESTION -

What is the PR value of the link that goes to my website?

Thanks in advance,

MAX