Verified answer

You should use "結成"

When people forms a organization or group, "結成する" is used.

ex.

form a band > バンドを結成する

form a union > 組合を結成する

establish a political organization > 政治団体を結成する

If you will google "バンドを形成", you will get tons of examples.

形成 = a shape, formation, transformation; to shape into something, to form into something, to transform into something.

結成: 結 means "a knot" as a noun, and means "to bind" or "to tie" as a verb. 結成 would then be "to combine", "to tie" or "to bind" different types of things together.

To form a band, you use 形成 in Japanese and either 形成 or 組成 in Chinese.

形成 formation of

結成 formation, combination, organization

I would use 形成

and it would be [object] を形成する。