How can I kick players with a custom message if they don't have the mods they need?

(obligatory noob question)

My server is modded and requires players to download a modpack in order to play. Not all of the mods are crucial and are client-side only. I only need to check for the crucial ones. If the player doesn't have all of these, they need to be kicked with a custom message, including a link to where the modpack can be downloaded.
I don't know much about plugins or stuff so how could I do this?
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