Simply append the RETURNING...
clause to the generated query:
const h = this.collection.$config.pgp.helpers;
const query = h.insert(values, null, 'branch') + 'RETURNING pk_branchID';
await this.collection.one(query);
You must have a large object there, if you want to automatically generate the insert. Namespace helpers is mostly valued when generating multi-row inserts/updates, in which case a ColumnSet is used as a static variable:
const h = this.collection.$config.pgp.helpers;
const cs = new h.ColumnSet(['col_a', 'col_b'], {table: 'branch'});
const data = [{col_a: 1, col_b: 2}, ...];
const query = h.insert(data, cs) + 'RETURNING pk_branchID';
await this.collection.many(query);
Note that in this case, we do .many, as 1 or more rows/results are expected back. This can even be transformed into just an array of id-s:
await this.collection.map(query, [], a => a.pk_branchID);
see: Database.map