Answer is Yes, you can use FLTK with closed source/proprietary code.

You can even statically link the FLTK library in with your closed source executable (something the normal LGPL doesn't allow, due one of the four LGPL2 exceptions, exception #⁠3, in the license file that comes with FLTK (historically named COPYING in fltk's source directory) which says:

3. Static linking of applications and widgets to the
   FLTK library does not constitute a derivative work
   and does not require the author to provide source
   code for the application or widget, use the shared
   FLTK libraries, or link their applications or
   widgets against a user-supplied version of FLTK.

Regarding "how do I do it", exception #⁠4 goes into that detail, which says:

4. You do not have to provide a copy of the FLTK license
   with programs that are linked to the FLTK library, nor
   do you have to identify the FLTK license in your
   program or documentation as required by section 6
   of the LGPL.

   However, programs must still identify their use of FLTK.
   The following example statement can be included in user
   documentation to satisfy this requirement:

       [program/widget] is based in part on the work of
       the FLTK project (https://www.fltk.org).

So for instance if your app has a "Help -> About" menu dialog, you can put the citation there,
or if it has a command line help flag (-h, -help), you can put it there, or put it in your app's written documentation.

For elaboration, please re-ask on the fltk.general google group.
And please, in the future, ask questions about fltk on that group so others besides devs can see replies and contribute answers.

Github Issues are just for bug and RFE reporting, are bad for questions like these because once the issue is closed, it becomes almost invisible to the public, and others can't as easily contribute or benefit from the discussion, as often non-devs have more interesting answers in the form of anecdotes and a variety of real world situations.

That all said, this is one of the things you'd bounce off your IP (Intellectual Property) lawyer, as anyone doing closed source should really be consulting for not only license compliance, but creating closed source licenses/warranty/etc for your app.

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