Hey
I hope I understood correctly... you could do it using a variable and a "switch" action.

Initialize a new variable called "SharePointValue" and set it to String. Leave the value empty for now.
Add a "Switch" action and as the "On" value you select your Forms Value from the Choice Field from the "Get response details" action.
Inside the switch, write the Form value in the "Equals" field and below it add a "Set variable" action. In that "Set variable" action you simple select the variable you created at the start (SharePointValue), and set its Value to the SharePoint Choice you want to add to SharePoint, if that Form Value has been selected. You can add multiple Switch "Cases" if you have multiple values you want to cross-reference.
Then in your "Create item" action, simply select the variable (SharePointValue) you filled in the Switch action. It's going to hold the value you want, not the one that's coming from the Form.
In my example, the Form has a choice value of "FormValueXYZ" and if the user selects that, in SharePoint the value would then be "SharePointValueABC".
Cheers