I have set the following: mygrid.setMultiselect("true"

have set the following: mygrid.setMultiselect(“true”);

put the grid into a form with a button to process multiple selected records.

When I
select multiple rows, then click the form submit form button, where do I find
the ids of the multiple rows I just selected?

Grid is not ready-to-use form
control. To pass selected rows to server on form submission you need to get
selected ids with script (getSelectedId()) onsubmit and put them into some
hidden form field.