The FLkey has both pad and pot Custom Modes which can send MIDI data as per your customisations in Components, for example you can edit pads so they can send note data or adjust momentary or toggling behaviour of control change messages.
When you hold 'Shift' and press the pads marked 'Custom' for the pot and pad modes, these custom controls can be used to manually map / link controls to 3rd party plugins and other controls in your FL Studio projects.
The process of editing custom modes is done in the FLkey 37 and FLkey Mini section of Components. This is virtually identical to these articles for the Launchkey range:
Components Launchkey Mini MK3 Guide
Components Launchkey MK3 Components Guide
After you've set up the custom modes as you like you can manually link the pots and pads in the Custom Mode to any control in FL Studio either via the 'Per Project Link' or the 'Global Link' method. I'll cover this process in more detail.
Per Project Link
These are MIDI links that are unique to each project and one control per function.
You can map any control of the custom modes to any parameter in FL Studio per project by tweaking the control you want to adjust in FL Studio and going to 'Tools' > 'Last Tweaked' > 'Link to Controller' and then adjusting the pad or pot to map this to that parameter.
This are links are carried over across FL Studio projects and may only be active when that specific plugin or window is in focus, allowing for one control to serve many different functions.
You can link any control like this by clicking the 'Multilink to controllers' button as per the screenshot below, then click the parameter you wish to control in the plugin or FL Studio window, and finally, move the pot or press the pad you wish to link to this parameter.
After you've made the link, right click the 'Multilink to controllers' button again and select 'Overwrite global links'. Move the control when the window comes up so it populate this with a permanent link which will carry over into new projects when this plugin is in focus.