Calculated field in form to update main table

I have a ‘DateReceived’ field in my table, as well as ‘DeadlineDate’ the ‘DeadlineDate’ will always be 21 days after the ‘DateRecieved’. I have no problem with creating an expression that will calculate this.
However, I want to create a functional form for other users in my organisation to use with ease. Essentially in the form, when I enter a value in the 'DateReceived' box, and tab to the 'DeadlineDate' box, i want the value to automatically update to 21 days later. I know how to do this, but I need this calculated value to be stored and updated in the main table. I don’t understand how this works, without running a make table query every time a new value is entered.

If I create the field in the main table 'DeadlineDate', as a standard date format, then import it into the form, and make the expression in that field in the form, will this translate to the main table?

I’m slightly confused and the only help I can find online, is to either create the calculation in the form only (meaning the table will not update), or create and run a query (which means running it every time a new value is entered).

Please Help