I'm assuming you mean adding it to the 'tension' information that is on each event by default. Simply click on the even you want to add tension information to. Then click on the inspector button (top right 'i') and you will find a tension box in the Values tab. Simply type your percentage as a whole number and that's it.
FWIW, I just had a look, and there is a tension setting in the values tab in some of my projects, and not in others. Something must select or activate it, but I can't figure what.
To create "Tension" from scratch (blank template): Go to Timeline Settings, then add a new "Properties" Name it "Tension" then give it a Type "Percentage", click OK to accept change and close the dialogue box, then open up Display Settings, in the "Other" section, drag and drop to left or right of the event columns. Click Save, to accept changes and close dialogue box. Then create an event, whatever date and time you want. Make sure the Inspector box is open when you then left click on the event to select it. Then click to select the "Values" tab in the inspector box. In the "Tension" section, enter a whole number -50, 25. 100. 245 to set the percentage as. No decimals. Negative or positive numbers are okay. Though for display purposes, anything 100% or mor, fills the bar the same. only the number changes.
THANK YOU!! I knew there had to be a way to do it!