Dynamic Date field to be set for each 20th based on form field

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HI all,

I have a process that will be opened for tax payment. By law, the payments need to be handled until each 20th day for each month. So, the idea is to create a reminder taking that date, so the user does not forget to make the payment.

I have a Date of Request from where I pick up the month and year, and I want to set the reminder date as 20th/Month of Request Date/Year of Request Date.
I would like to have this automatically calculated instead of relying on the user to select something like Due Date.

Does anyone have ideas on this?
Thank you!