Query and update another table for default value for field

Hello

I'm trying to work out how to do this and not having much luck...

So I know I can set the default value of an editor field (https://editor.datatables.net/reference/option/fields.def)

But I need to get that value from another table in the database - and, actually, increment that value in the other table too.

So the table I'm editing is "projects"... it has a "job number" field.

That job number is in another table, "networks" which keeps track of the next job number for the particular network.

How do I get the data from the other table (and update it too!)

Many thanks

Ken