The spec says whether substitutions are allowed and when. Many projects accept requests only before bid day, up to a set deadline.
A supplier may offer a different brand of operable partition than the basis of design. It must show that the product matches the specified performance.
Read the instructions to bidders and Division 01 for the procedure. Use the required form. Attach product data that compares your product with the one specified.
Approval before bid comes by addendum. If you price an unapproved product, say so in your proposal. The GC carries a risk if you stay silent.