You can use the openargs parameter of the OpenForm method to pass in the strWhere string.
In FormB, use the OnOpen event to get the parameter passed in by referencing me.openargs. Then set the subforms filter property with the where string.
ex. for FormB
me!subform.form.filter = me.openargs
me!subform.form.filteron = true
Let say the subform in FormB has a record source of Table1
you could do this in FormB:
me!subform.form.recordsource = “select * from Table1 ” & me.openargs