You have to use send, as you wrote. "To send" is an infinitive. "To sent" is meaningless.
We are required is present simple passive.
"Required" is not an adjective. It is part of the passive voice. The verb "are" makes it present tense.
How is it passive voice. In earlier posts it was mentioned that this is not passive voice. Please explain.
Which post was it?
I think 'not in the passive voice' in post#4 refers to 'Please let us know' rather than 'we are required'.