You will need to call MS to do the transfer and to make the reservation. Neither transaction can be done online.
If you cancel that reservation (which you could do online), the transferred points will not show in your online account. You will need to track them yourself and also have to call MS when you want to use them for something else.
For those who may not already know, this is the reason so many of us do not recommend owning two different memberships (unless you very rarely need to combine points to make a reservation). It can be frustrating to book, and involving MS means online bookers have at least an hour's head start over you. Sometimes, that means what you want is taken by someone else before MS even opens.