Enter it as cash under Schedule C. Since you did not get a form, you will need to reconstruct your income. Look at your bank statements. Income from Jan1-December 31, only.
Even without any tax forms, you would enter the income as self-employment income. You would need to file a Schedule C and pay SE taxes. You may also enter any expenses related to your work to reduce your income.
To file a Schedule C, here are the steps:
Sign in to your account and select Pick up where you left off
At the right upper corner, in the search box, type in schedule C and Enter
Select Jump to schedule C
Follow prompts
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If you did not receive a 1099-K, you will choose "Other Self-employed Income". The examples listed for cash and checks are types of income included but it is not limited to just cash and checks. See screenshot below: