Creating a dropdown list to select by different attributes in QGIS

This solution assumes the following:

• You are using QGIS 3
• That you are looking for a basic toolbar solution
• That macros are enabled (Settings > Options > General)
• That this is for a specific QGIS project file
• That the values you want to search by in the layer are only used once
• The layer is not nested in a group layer
• You have some familiarity with Python 3

As is, this script will unselect any existing selections from the given layer, select the feature that matches the chosen value from the dropdown list, zoom to it, and clear the selection. It happens fast enough, you won't see the feature be selected and unselected.

In my example below, I am using a layer called 'corps' and the field I use is called 'name'. Adjust to your case.

In Project > Properties > Macros, you can adjust the openProject section to this:

def openProject():
    from qgis import utils
    from qgis.utils import iface
    from PyQt5.QtWidgets import QLabel, QComboBox, QPushButton
    from qgis.core import QgsProject,QgsLayerTreeNode,QgsLayerTreeGroup

    # VARIABLES TO CHANGE
    #####################
    layername = 'corps'
    field = 'name'
    #####################

    canvas = iface.mapCanvas()
    layer = QgsProject.instance().mapLayersByName(layername)[0]

    toolBar = iface.addToolBar("Zoom To Feature")
    toolBar.setObjectName("Zoom to Feature")
    lbl1 = QLabel(iface.mainWindow())
    cbo1 = QComboBox(iface.mainWindow())
    btn1 = QPushButton(iface.mainWindow())

    # Setting up toolbar
    lbl1.setText('Zoom to: ')
    btn1.setText('GO!')

    tbAction = toolBar.addWidget(lbl1)
    tbAction = toolBar.addWidget(cbo1)
    tbAction = toolBar.addWidget(btn1)

    toolBar.setEnabled(True)
    cbo1.clear()

    features = layer.getFeatures('1=1')
    flist = []
    for feature in features:
        flist.append(feature[field])
    flist.sort()
    cbo1.addItems(flist)

    btn1.clicked.connect(lambda: zoomtofeature(field, cbo1.currentText()))

    def zoomtofeature(field, value):
        iface.setActiveLayer(layer)
        layer.removeSelection()
        features = layer.getFeatures(f"{field} = '{value}'")
        ids = [i.id() for i in features]
        layer.selectByIds(ids)
        canvas.zoomToSelected(layer)
        layer.removeSelection()
    
    iface.addToolBar(toolBar)

This puts together toolbar elements, loads the values found in the 'name' field, and displays the toolbar.

At this point, if you open another project from this window, the toolbar will persist. Add the following to the closeProject section of the macro script to remove the toolbar as it closes.

def closeProject():
    parent = toolBar.parentWidget()
    parent.removeToolBar(toolBar)

Once you save the project file, this toolbar should only appear when the project it is saved to is open.