How to set Header in htaccess?

Question

Apache HTTP Server

Hello.
I have this htaccess rule:

RewriteRule ^sessions/(.*?)$ http://somesite/sessions/$1 [QSA,P,L]

I would like to pass the Headers I need with this proxy request.
I do not want to make a bicycle on Curl.
Googling yielded no results. I tried all the options - the headers I need are not transmitted.
How to proceed?