You can quickly access your list of companies by clicking the arrow next to the budget name. 

Then press on the Company List icon (Second RED square).

Then press on ADD to create a new company: 

Fill out the form and press on SAVE once you are done.

Don't forget to share the budget with your client

Related Articles (Please refer to the links below)

-How to access my list of companies and the companies shared with me?

-How to share a company

-What are the different sharing permissions? 

-How to change sharing permission?

-Select plans for individual company in a multi-company account