To create the Lost Cousins event in webtrees follow the procedure below:

View the person you wish to edit

Select the "Facts and Events" tab 

Locate the "Fact or Event" row and select "Custom Event" from the dropdown control.

Click the "Add" button.

On the form that appears you can leave all the fields blank except for the following

For "Type" you should put "Lost Cousins" (without the quotes)
For "Date" you should put the census year (e.g. 1881)

Click the "Save" button at the bottom left of the form.

To export a GEDCOM in webtrees

Select "My Page", "Administration"

Click on "Family Trees" on the left of the screen.

Click on "Download" next to the tree you wish to export.

Click "continue" and then save the file on your local computer.