When defining trigger in a declarative pipeline, it will suffice to write

pipeline {
 agent any

 triggers {
   cron('H H(0-7) * * *')
 }
}

and build once. 

After the first build accessing the job config UI will show the trigger configuration.

However if you do the same thing with a scripted pipeline

node {
    properties([pipelineTriggers([cron('H H(0-7) * * *')])])
}

you need two builds to register the trigger (it won't appear in the UI and won't be working until the second manual build).