You can definitely do it but 20$ is outrageously low. Some projects do allow you to put a bounty on feature requests. But the feature you’re asking sounds non-trivial. The work to implement it includes getting familiar with the project, coding the feature, testing it, writing automated tests, updating documentation, opening the pull request for review and making changes requested by the maintained during review. You’re looking at several hours/days of work. 20$ for this amount of work is a small tip.