How to download games of a player with the api?

Dear all,
In an older post I found this site (https://lichess.org/api#operation/apiGamesUser) with different APIs to download games for an user, but I don't understand how I can add the query parameters into the main API.
I want to use the API https://lichess.org/api/games/user/{username} in combination with the query parameter "since". How can I do this?
Best regards,
Jan

@BlunderJan

for your games since 2020-06-01 00:00:00.000 GMT it is:
https://lichess.org/api/games/user/BlunderJan?since=1590969600000
or since 2020-06-01 00:00:00.000 and until 2020-06-10 23:59:59.999
https://lichess.org/api/games/user/BlunderJan?since=1590969600000&until=1591833599999

to convert a date into a timestamp you can use something like this:
https://www.epochconverter.com/

Perfect :-) Thank you very much @kleineme !!!