To add a new form to a channel of a Team, go to the channel, and click the + on the tab bar.
In the pop-up window, click the "Online Form".
To create a new blank form, in the next page, select the 2nd option from the left menu. then fill in the form name and form ID. Click "Save" button to add this new form to the Tab.
To create a new form from the template library, in the next page, select the 3rd option from the left menu. then select the form template you'd like to use from the template list on the right.
Click "Save" button to add this new form to the Tab.