I need to use el mandato formal using the usted command
The form is identical to the 3rd person singular of the present subjunctive. For regular verbs, if they end in -ar, replace the ending with -e; otherwise, replace -er or -ir with -a. Examples:
entrar --- entre
beber --- beba
vivir --- viva
I'm not sure what you mean, but I'm guessing it's just when you use the Usted form of a verb as a tu command!