Hi,
you have two possible main paths:
-the MC's fetish is either discovered or appears to be obvious.(revealed)
-The Girlfriend reveals herself as having the same kink or is discovered. (or discovers it herself)
How to create such discoveries?
either through elements more or less visible i.e. books, pictures, drawings, sculpture,(computer screen - browser historic ...)
the classical diary, ...
or through events i.e. halloween (a classic for fetish outfits) - a revelation (punished at school - spanked by an evil
teacher (witnessed by GF) - a "stupid" dare one boozy night - a mix of dare and revelation i.e. MC loses a bet
to GF and she proposes 2 options like - running around the block naked OR letting her spank him (he likes it a lot... like really a lot)
(I mostly considered YA themes - but you can almost transpose to older people without major difficulties, replacing a schoolroom with an office one - innocence with greed and good grades by money or power)
the main difficulty resides in the credibility of the scene - truth or dare having been used to death maybe let it rest for a while.
in my opinion, if you consider a YA scenario, credibility is added if the plot is exterior to them - they are
accessories to it (not Forced - unless you want it so - but more like set in a collision course toward a revelation)
i.e. GF goes to swimming pool - bra gets soaking wet - GF goes back to class braless - MC loses it over her tits (like tex avery's wolf)
(yeah i know it lacks real creativity it's merely an example)
(you can switch bra to panties if you prefer)
(or have the GF being caught kissing another girl in the shower, then having to justify her heterosexuality (in kinky ways))
(or thru an evil 3rd party (teacher counsellor nurse sheriff whatever) the poor YAs are pushed towards an incredibly kinky future)
(possibly resisting it at first but discovering thru resistance that they actually enjoy it)
if the MC wants the GF to either discover or wants to lead her to his desired goal, the best way is bread crumbs... (good for a slow burn corruption patreon milking game)
(and most girls IRL are too curious for their own good anyway)
another way is the "revenge" plot: GF wants to punish the MC (or the reverse) (good for humiliation petplay and so on)
I won't go over the whole jealousy, cheating, swinging, bisexuality, FemDom, FinDom and the abominably cursed NTR themes - you'll pick what you like for your story.
Last point, if your topic is about how to have a fetish in a game without having the game Tagged with it, i guess you can't.
a lot of people are hardcore proper tags fetishists and will ask for your game to be properly tagged.
(and will use spoilers to have their way - so bye bye surprise effect)
sky is the limit (sky being your imagination) (or kinkyness)
GL