Video posts are included in Premium Plus Events. If you have the Premium Plus plan, tap the '+' add button, and then tap 'Videos', as shown below.
If you don't have Premium Plus, the host or a guest can also activate the Video posting feature on the Trial, Standard and Premium Plans. To activate the Video feature on a Trial, Standard or Premium plan, click the '+' button to post. Note the Video shows locked.
Click on the 'Videos' button, and the following window will be displayed. Click 'Select' to add the Video feature. The $10.00 fee is a one-time fee for the duration of the Event.