The affirmative tú command of the verb escribir is escribe.
When giving commands in Spanish, particularly with the informal second person (tú), the endings of the verbs change depending on whether they are -ar, -er, or -ir verbs. Since escribir is an -ir verb, the affirmative tú command uses the third-person singular form of the present tense. For example, in the present tense, ‘tú escribes’ (you write) becomes ‘escribe’ when you’re instructing someone to write.