It's a little bit early to signal intent to participate in specific topics.
I assume you've seen Gathering ideas for GSoC 2021! where we say that Swift intends to participate in GSoC this year. Note though that this isn't really confirmed yet and the intent of the post was to gather more ideas, in case the community had any new project ideas they'd like to start fleshing out early.
As for applying for participating in a specific project, such as the argument parser interactive mode, mentored by @nnnnnnnn, the right timeline to apply for this will be from March 30th, as outlined in the important dates in the other post:
Nate may be able to answer any questions you have in the meantime, but generally speaking applying to projects will be done from March 30th, and you'll have to create an account on https://summerofcode.withgoogle.com and apply though there then.
In the meantime you can also read the Guide published by GSoC which should help you understand how this entire process will work: What is Google Summer of Code? | Google Summer of Code Guides
Hope this helps,