In treeherder, select the job you want to re-run (Note: it currently only works for [https://searchfox.org/mozilla-central/source/taskcluster/taskgraph/actions/retrigger_mochitest.py#30-31 some jobs]). The details panel should open. Look for the 3 dots icon ("Other job details") and select "Custom actions...".
[[File:Custom-action.png|framethumb|Selecting "Custom Action..." in treeherder.]]
The <tt>retrigger-mochitest</tt> action (TODO: A better name is needed) is the one selected in the drop-down menu. Modify the task parameters as desired and select "Trigger".