1. Select the arbitrary ReactionLink icon from the tool bar.
2. Select the target SpeciesAlias as a Modifier, then select the ReactionLink for the target base.
The connection point(s) will be highlighted as blue when you place the cursor over the target ReactionLink.
As there is no logical distinction with the point of the connection, connect at a favorite place for visually convenience.
It is possible to connect arbitrary numbers of ModificationLink(s) to one BaseLink.
See Also: Insert Multiple Lines
The connecting points on the BaseLink are a process node.

When ModificationLink is added, in respect of the logic structure of a model, there are no increase and decrease in the number of <Reactions(es)>, and it becomes a part of Reaction which connected BaseLink expresses.
You can check which Modifier of the Reaction being added by clicking the target Reaction from the <Reactions> list window to pop-up the dialog window.