There are two ways you can try:
1) Change the threshold will easy trigger this alarm (for testing) without actually creating a lock (see attached example)
2) The second way is creating a lock:
The threshold of "Locks - Blocked Processes" is 120 seconds by default, please find attachments.
And the background schedule for collecting data is every 2 minutes as well. You can update them for your need.
You can trigger the alarm intentionally by SQL script:
1. Open one window in SSMS, run the first query
BEGIN TRANSACTION
SELECT * FROM YourTable WITH (TABLOCKX, HOLDLOCK)
WAITFOR DELAY '00:02:00' -- 2 minutes
ROLLBACK TRANSACTION
2. And then in another Query Window:
SELECT * FROM YourTable