It doesn't specify what type of action is needed to use Forgotten Trick, so we need to fall back on the Combat chapter of the core rules.
Extraordinary Abilities (Ex)
Using an extraordinary ability is usually not an action because most extraordinary abilities automatically happen in a reactive fashion. Those extraordinary abilities that are actions are usually standard actions that cannot be disrupted, do not require concentration, and do not provoke attacks of opportunity.
Seems that using Forgotten trick is a standard action...
There is also an alternative rules interpretation based on the Ninja Ki Pool ability.
Ki Pool (Su)
At 2nd level, a ninja gains a pool of ki points, supernatural energy she can use to accomplish amazing feats. The number of points in the ninja’s ki pool is equal to 1/2 her ninja level + her Charisma modifier. As long as she has at least 1 point in her ki pool, she treats any Acrobatics skill check made to jump as if she had a running start. At 10th level, she also reduces the DC of Acrobatics skill checks made to jump by 1/2 (although she still cannot move farther than her speed allows).
By spending 1 point from her ki pool, a ninja can make one additional attack at her highest attack bonus, but she can do so only when making a full attack. In addition, she can spend 1 point to increase her speed by 20 feet for 1 round. Finally, a ninja can spend 1 point from her ki pool to give herself a +4 insight bonus on Stealth checks for 1 round. Each of these powers is activated as a swift action. A ninja can gain additional powers that consume points from her ki pool by selecting certain ninja tricks.
If you read the last two lines of the quote above as including the "additional powers" you select as part of the "each of these powers" group, then all ninja ki powers default to a swift action unless otherwise stated. I don't think this interpretation is correct based on the wording, but I see other forum goers have mentioned it in previous discussions of Forgotten Trick.
All that said, this ability is pretty lame if you have to spend a standard action for it. Because of how expensive this ability is and how limited of a time you have to make use of it, I'd say making it a free action is a fine house rule.