Learn Finnish: Linkki on täällä, mutta sen klikkaaminen on vaikeaa. - The link is here, but clicking it is difficult.

It comes from the verb klikata (to click). To make a verbal noun (similar to adding "-ing" in English), you take the strong vowel stem of the verb, which is klikkaa-, and add the suffix -minen. This turns the action into the noun "clicking".

Why does the sentence use sen (the genitive of "it") instead of a regular object case like sitä?

Because klikkaaminen functions as a noun here, it cannot take a normal direct verb object. Instead, the target of the action (the thing being clicked) is put into the genitive case. You can think of it as literally saying "its clicking" rather than "clicking it".

Why is vaikeaa in the partitive case instead of the basic nominative vaikea?

When the subject of a sentence is an abstract action or concept—in this case, the act of klikkaaminen (clicking)—the describing adjective that follows the verb olla (to be) must be in the partitive case.

The verb klikata looks like an English loan word. Does it behave like regular Finnish verbs?

Yes! It is borrowed from the English word "click", but it is perfectly adapted into a standard Finnish Type 4 verb (verbs ending in -ata / -ätä). Just like haluta (to want) gives the stem halua-, klikata gives the stem klikkaa-.