have you looked at the documentation samples:
once you have a reference to
map, you can use it as documented in leaflet documentation.
I have not looked myself, but have you gone digging through the code for the isoline plugin:
You might find there are parameters you might manage to configure; or you might be able to make your own custom variant to use in place of this ootb plugin.
ah, so the address is https://www.sääneuvos.fi/test/testAPI4.php.
when i try load that page, i see the js console is reporting the following error:
Windy API used on unauthorized domain
Have you authorized your domain (www.sääneuvos.fi) in your API config (https://api4.windy.com/api-key/)? If you have, ivo's code might be having issues with the special characters in your domain name (ä)...
start with the samples/tutorials at https://api4.windy.com/. just copy the sample html as is into your own html file, put it on your web server, and look at the resulting page. make changes as needed. the samples cover a lot of ground in terms of typical changes folks might be interested in.