So you might legally be somewhat limited in what you can write there (like "donate to me SO THAT I can write more code YOU enjoy"), but saying
"Support me / donate to me / Support my work through Ko-Fi"
is entirely fine.
I run a free web service where some users have asked me to set up a donation system but AFAIK it is illegal so I haven't done it. Instead I set up a company and will sell premium accounts with some borderline useless but actual feature that normal accounts don't have, so that it's no longer a donation.
AFAIK, just adding your PayPal details or bank account number to your website would be okay because you would not be making any sort of an appeal.
"Minua voi tukea tämän linkin kautta"
"Lahjoita minulle tämän linkin kautta"
All are acceptable.
And since "lahjoita" is entirely equal to "support" then also
"Tue minua tämän linkin kautta" is entirely fine.
Of course the Finnish justice system is known for being unjust and arbitrary in many cases so your mileage may vary.
I also run a donation based site (which also has ads) and I declare both of these on the form and pay my share of taxes from it. But it is then fully legal money and I can do whatever I like with it from government perspective.
I don't speak any Finnish at all, but if the wording is so important, wouldn't it work if you ask the money to pay for the services cost instead? Not as a donation to yourself. Servers cost, your time costs etc. Could all be counted as an upkeep of the project.
I don't think it's a worthwhile risk to take for Finnish open source developers to accept donations and think it won't be considered illegal by the authorities.
https://rescue.fi/article.php/20070131_effi
It was only district court but the prosecutor did not seek the court of appeals (which they legally have to do if there's a chance their view might succeed), leaving the precedent.