public function getPrSolicitation()
{
return $this->hasOne(Tblprsolicitations::className(), ['pr_solicitation_id'
 => 'pr_solicitation_id']);
}

public function getPRsuppliers()
 {
  return $this->hasOne(Tblprsuppliers::className(), ['pr_solicitaion_id'
  => 'pr_solicitaion_id']);
}

<?= $form->field($model, 'prSolicitation[pRsuppliers][supplier_id]')->textInput(['placeholder' => 'Enter supplier'])->label(false); ?>

public function search($params)
{
    $query = Tblpritems::find();

    // add conditions that should always apply here

    $dataProvider = new ActiveDataProvider([
        'query' => $query,
    ]);

    $this->load($params);


    // grid filtering conditions
    $query->andFilterWhere([
        'PRlineID' => $this->PRlineID,
        'Quantity' => $this->Quantity,
        'Unit_Price' => $this->Unit_Price,
        'Extended_price' => $this->Extended_price,
        'Currency_ID' => $this->Currency_ID,
        'PRID' => $this->PRID,
        'pr_solicitation_id' => $this->pr_solicitation_id,  //This is what am using t filter the grid
        'date_item_received' => $this->date_item_received,
        'Quantity_received' => $this->Quantity_received,
        'Received_by' => $this->Received_by,
        'item_received_status' => $this->item_received_status,
    ]);

    $query->andFilterWhere(['like', 'Tracking_Code', $this->Tracking_Code])
        ->andFilterWhere(['like', 'Description', $this->Description])
        ->andFilterWhere(['like', 'Remarks_on_receipt', $this->Remarks_on_receipt]);

    return $dataProvider;
}

<?= $form->field($model, 'prSolicitation->pRsuppliers[supplier_id]')->textInput(['placeholder' => 'Enter supplier'])->label(false); ?>