You may do this with a phone call but a written record will help avoid future misunderstandings.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
We are interested in leasing office space at <address of property>. The location is ideal for our business and we are impressed with how well you've maintained the building.
We are interested in a <length of lease> lease. Please advise us of space availability and lease terms as soon as possible. We look forward to doing business with you.
encl: <List of enclosed items goes here>