If you're fetching a single item only then, you need use select before your FirstOrDefault()/SingleOrDefault(). And you can use anonymous object of the required properties.
var name = dbContext.MyTable.Select(x => new { x.UserId, x.Name }).FirstOrDefault(x => x.UserId == 1)?.Name;
Above query will be converted to this:
Select Top (1) UserId, Name from MyTable where UserId = 1;
For multiple items you can simply chain Select after Where:
var names = dbContext.MyTable.Where(x => x.UserId > 10).Select(x => x.Name);
Use anonymous object inside Select if you need more than one properties.