How to save ManyToOne realtion by ID in Spring Boot JPA

My application contains one class that looks like this:

import javax.persistence.*;

@Entity
public class PerformanceIndicator {

  @Id @GeneratedValue private Long id;

  private String name;
  private String description;
  private int level;

  /**
   * @JoinColumn says that Performance Indicator table will contain a separate column
   * ARCHITECTURAL_LAYER_ID which will eventually act as a foreign key reference to primary key of
   * Accountability Model table. @ManyToOne says that multiple Architectural Layers can refer to
   * same Accountability Model (Multiple Architectural Layers can be registered in same
   * Accountability Model). Additionally , with optional=false we make sure that Architectural Layer
   * can exist without a Accountability Model.
   */
  @ManyToOne()
  @JoinColumn(name = "activity_id")
  private Activity activity;

  @ManyToOne()
  @JoinColumn(name = "architectural_layer_id")
  private ArchitecturalLayer architecturalLayer;

  public PerformanceIndicator() {}

  public PerformanceIndicator(String name, String description, int level) {
    this.name = name;
    this.description = description;
    this.level = level;
  }

  public Long getId() {
    return id;
  }

  public void setId(Long id) {
    this.id = id;
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public String getDescription() {
    return description;
  }

  public void setDescription(String description) {
    this.description = description;
  }

  public int getLevel() {
    return level;
  }

  public void setLevel(int level) {
    this.level = level;
  }

  public Activity getActivity() {
    return activity;
  }

  public void setActivity(Activity activity) {
    this.activity = activity;
  }

  public ArchitecturalLayer getArchitecturalLayer() {
    return architecturalLayer;
  }

  public void setArchitecturalLayer(ArchitecturalLayer architecturalLayer) {
    this.architecturalLayer = architecturalLayer;
  }

  @Override
  public String toString() {
    return "PerformanceIndicator{"
        + "id="
        + id
        + ", name='"
        + name
        + '\''
        + ", description='"
        + description
        + '\''
        + ", level="
        + level
        + ", activity="
        + activity
        + ", architecturalLayer="
        + architecturalLayer
        + '}';
  }
}

And architectural layer class:

import com.fasterxml.jackson.annotation.JsonBackReference;

import javax.persistence.*;

@Entity
public class ArchitecturalLayer {

  @Id @GeneratedValue private Long id;

  private String name;
  private String description;

  /**
   * @JoinColumn says that Architectural Layer table will contain a separate column
   * ARCHITECTURAL_LAYER_ID which will eventually act as a foreign key reference to primary key of
   * Accountability Model table. @ManyToOne says that multiple Architectural Layers can refer to
   * same Accountability Model (Multiple Architectural Layers can be registered in same
   * Accountability Model). Additionally , with optional=false we make sure that Architectural Layer
   * can exist without a Accountability Model.
   */
  @ManyToOne
  @JoinColumn(name = "accountablility_model_id")
  @JsonBackReference
  private AccountablityModel accountablityModel;

  public ArchitecturalLayer() {}

  public ArchitecturalLayer(Long id, String name, String description) {
    this.id = id;
    this.name = name;
    this.description = description;
  }

  public ArchitecturalLayer(String name, String description) {
    this.name = name;
    this.description = description;
  }

  public ArchitecturalLayer(Long id) {
    this.id = id;
  }

  public Long getId() {
    return id;
  }

  public void setId(Long id) {
    this.id = id;
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public String getDescription() {
    return description;
  }

  public void setDescription(String description) {
    this.description = description;
  }

  public AccountablityModel getAccountablityModel() {
    return accountablityModel;
  }

  public void setAccountablityModel(AccountablityModel accountablityModel) {
    this.accountablityModel = accountablityModel;
  }

  @Override
  public String toString() {
    return "ArchitecturalLayer{"
        + "id="
        + id
        + ", name='"
        + name
        + '\''
        + ", description='"
        + description
        + '\''
        + ", accountablityModel="
        + accountablityModel
        + '}';
  }
}

When I try to save a PerformanceIndicator object that already has a existing I want to do this by it's ID. But there also have to be an option to create an Activity and Architectural layer if the users wants to.

for example:

PerformanceIndicator p = new PeformanceIndicator();
p.setName("test");
p.setDescription("test");
p.setLevel(1);
p.setActivity(1);
p.setArchitecturalLayer(1);

performanceIndicatorService.save(p);

What can I do to be able to persist this by passing an ID but also be able to pass in a object of the Activity/ArchitecturalLayer?