No preposition is necessary in the following:
I'm a C#/Js/Python developer with 10+ years of experience creating software at corporations and cutting-edge startups.
Experience in (gerund) is a common usage in English. In this list of common combinations of nouns followed by prepositions and gerunds, both experience in and experience without a preposition are acceptable:
She has a great deal of experience in introducing new products to international markets.
With the noun "experience," sometimes a gerund is added without the preposition "in." "Experience introducing new products" would also be acceptable.
As an illustration, look at these book blurbs which feature the exact phrase "ten years of experience creating ..."
