Hi, Mark. As the sentence is, you'd use "came". However, giving you a more grammatically oriented answer, the word "just" would make me recommend changing "I just" to "I've just". In this case, though, the answer would be "come". I've just come to know that... I hope that is useful to you. Regards.

came

Thank you again, sir!

Honestly, Mark, that depends on how strict you want to be in terms of grammar rules. There is this nice discussion in Word Reference about it: https://forum.wordreference.com/threads/just-past-simple.268652/ Basically, "just" is mostly used with the Simple Past (came) in American everyday conversation. Some people on this thread, however, mention it is gaining ground in England, too.