The easiest way its download my cheat event. This is the link :
https://github.com/DoSzero/Victoria-2-Population-increases-Cheat-Event-

Steps:
1 ) You need to select: Victoria 2 Only MAIN EVENT and change or adds files
2) Open Victoria 2\common and add in this event_modifiers.txt
3) Victoria 2\events
4) Start game and Press in keyboard tilde ( ~ , ₴ , Ё )
5) Write: event 80777
6) Waite 1 games day (roughly 1 to 5 sec real-time )
7) It's all yours population grow up 1250 %


Before
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After


You don't want to download something?

You can use a standard Event: event 14680 , but its population increases only by 1% because you need to spam this event.