If you want to do this via API, you can use the createCardRelation mutation. Here is an example of how to use it:

mutation {
  createCardRelation(input:{
    childId: CHILD_CARD_ID,
    parentId: PARENT_CARD_IS,
    sourceId: CONNECTION_FIELD_INTERNAL_ID,
    sourceType: "Field"
  }) {
    cardRelation {
      id
      childId
      parentId
    }
  }
}

You can check more details about the mutation in our developers documentation or using our interactive playground.