when check SelectedBranchId bind outside for post it is become null

Question

I work on razor asp.net core .i face issue selectedbranchid is inside post form form is get selected value without any issue

but if i use another post and get selectedbranchid it become null why 

so How to solve issue please 

on page model

 [BindProperty]
        public string SelectedBranchId { get; set; }

<div id="ShelfLabelPrintrSetupAdcSupport" style="margin-top:20px;">
        <form id="FrmShelfLabelPrintrSetup" method="post" onsubmit="return validateForm() ">

            <div class="row">
                <div class="col-lg-12 col-12 row">
                    <div class="col-md-3 col-lg-2">
                        <label for="branch-selectlbl" style="margin-left:3px;font-size:15px;font-family: 'Open Sans', sans-serif;font-weight: bold;">Print Server</label>
                  
                        <select id="branch-select" asp-for="SelectedBranchId" name="selectbranchid" class="form-select" style="margin-left:3px;font-size:15px;font-family: 'Open Sans' , sans-serif;font-weight: bold;" onchange="toggleButtonVisibility()">
                            <option value="0">--Select--</option>
                            @foreach (var branches in Model.branches)
                            {
                                <option value="@branches.iBranchCode">@branches.vBranchDesc</option>
                            }
                        </select>
                       
                    </div>
                    <input type="hidden" id="SelectedText" name="SelectedText" asp-for="SelectedBranchId" value="SelectedBranchId" />
                    <div class="col-md-3 col-lg-2">
                        <br>
                        <button  id="Searchtxt" type="submit" name="searchButton" value="search" asp-page-handler="Search" style="width:100px;margin-top:7px;" class="btn btn-primary">Search</button>
                    </div>     
                </div>
            </div>
        </form>

    </div>
</div>

<button id="createTxt" data-toggle="modal" data-target="#CreateshelflabelModal" style="width:100px;margin-top:7px;margin-left:7px;display:none;" class="btn btn-primary">Create</button>

now i will check another post method not related to form or another form different with post

public ActionResult OnPostCreateUsers(string selectbranchid)
{
    string selectedbranchid=SelectedBranchId;  //here when check it become null
}

why bind property outside form become null and how to solve this issue