code: public void setBillingSchedule(com.google.api.ads.admanager.axis.v201808.BillingSchedule billingSchedule) {
          this.billingSchedule = billingSchedule;
      }
docs: Sets the billingSchedule value for this Proposal.
  
  @param billingSchedule   * The billing schedule of this {@code Proposal}. This attribute
  is optional and
  default value is {@link BillingSchedule#PRORATED}.
  Either this attribute
  or {@link #billingCap} will be used, according to
  the {@link #billingSource}.
  This attribute can be configured as editable after
  the proposal has been submitted.
  Please check with your network administrator for editable
  fields configuration.
  <span class="constraint Applicable">This attribute
  is applicable when:<ul><li>not using programmatic, using sales management.</li></ul></span>