When creating a trigger step, select the option under 'Node selection' for 'specified script evaluates to true'.
According to the information below the input box, the 'node' variable should be set to the grid node being tested.

I will have multiple agents specified, and I want this step to run on a specific agent.
I do not want to have to worry about the port number, that is required in the 'On a specific agent' selection.

I put the following lines in my script, and got the following results

script:
${logger.info(node.getHostName())}
false

results:
<name of server>
<name of server>

I was expecting one of the iterations to have the name of my agent in the results.