Alpha Centauri 1 link alien artifact to network node

Solution 1:

There are two options:

1. Upon completion building the Network Node, activate the Alien Artifact and press Ctrl+H (Set Home City). The game will suggest using the Artifact. Note: this works even if the unit has no moves remaining (e.g., it was Held in a city and passed its move).
2. When the Alien Artifact activated, just step away from the city and step back into the city. Since usually, each city has at least one road around, the move will take only 2 * 1/3 of Artifact's moving capacity.

Also, SMAX (Alpha Centauri Alien Crossfire) automatically unholds Alien Artifacts in the city upon building the Network Node or starting building a special project. I'm not sure if the older SMAC works the same way.
Note: if unheld, the unit may have already skipped its move, but it still can do Ctrl+H; see above.

Solution 2:

I've just found out how to do it. I had the alien artifact on hold on the city in preparation for the completion of the network node. If you activate the unit, the next turn the game will act as if you've moved the alien artifact onto the city. Then the game will ask you if you want to link the alien artifact to the network node.