At the outset: just can mean "only" or it can have a temporal meaning, "a moment ago" or "not long ago", or, with some verbs, "recently". I don't believe that want is one of those verbs, except in very rare situations. To say "I have just wanted..." makes semantic sense only if you're recounting your mental state moment-by-moment.
We can use the present perfect with want to indicate that the want has existed recently and has continued to exist up to the present moment:
I have wanted our paths to cross for some time now.
In its sense of "a moment ago", just is incompatible with the present perfect because "just" situates the action in the past, albeit the recent past, not along a continuum that extends from the past up to the present moment. A time phrase in a present perfect construction cannot exclude the present.
I just saw him a moment ago.
I have just seen him a moment ago. ungrammatical
P.S. If the semantics of the sentence allow for the meaning recently, then just can be used with the present perfect, provided there are no other time phrases that relegate the action to the past:
I have just come from there.
I have just come from there an hour ago. ungrammatical
I have just seen her.
I have just seen her a second ago. ungrammatical