Granting Create Table permissions to a specific schema requires, granting create table at the database level and alter at the schema level (doing no grants to the dbo schema).

Create user [some user] from external provider

go

Grant alter,select,insert,update,delete on schema::[schema name] to user [some user]

go

-- if control or references is needed

-- grant control,references on schema:: [schema name] to user [some user]

grant create table, create view, create function,create type to [some user]

go

Always confuses me to remember the grant alter and the create table portions.