Glad you manage to make it work 😉 Flex part editor is not the easiest to work with but it is functional.
Normally just clicking on the control points or handles shall be enough. Which browser are you using?

I'm using Chrome. I did manage to create flexible parts but I noticed that the "subdivide" option doesn't work (or I don't know how).

Also I'd like to report another bug (if that's okay) that the export option crashes the page. Not always though, so that may be a problem on my end.

You need to select 2 control points that are side by side - Click the 1st point -> hold Ctrl key -> click the 2nd point
A new point will be created in between after you clicked the "Divide" button.

Regarding the other issue, it can be because of the size of the model and the format you chose.
How many parts does the model include? Which format do you try to export? Did you check the logo option?

Generally the best solution is to export in Collada with the "Group by Geometry" option. This one works even with huge models and produces smaller files which are easier to handle by the browser.

my model has around 600 parts and you are correct, exporting it grouping by geometry works.

Thanks so much for your answers.

No worries. You are welcome.