When you are invited to Partner Portal you will receive initial password generated by the system. Use it to login to "pp.salestrekker.com". IMPORTANT NOTE: access for Partner Portal url is different than for other users - so in case you try accessing at "app.salestrekker.com" login won't work.
Once you are logged in go to top right corner click on your picture or initials, choose Settings and go to "My password and security". Paste initial temporary password in "Current password" field and bellow type and confirm new password you want to use for login.