When choosing a product from the selection menu, be sure to click on the desired product and then focus on the quantity input field. If you are having trouble using the ref attribute, check out the following link for more information:
<el-select
ref="select1"
v-model="EntradaProducto.ProductoSucursalId"
filterable
style="width: 50%"
remote
@change="ChangueSelectProduct"
placeholder="Type in the name of the product"
:remote-method="remoteMethodSearchProduct"
:loading="loadingSearchProduct">
<el-option
v-for="item in optionsProducts"
:key="item.id"
:label="item.descripcion"
:value="item.id">
<span style="float: left">{{ item.marca +' | '+ item.descripcion }}</span>
<span style="float: right; color: #8492a6; font-size: 13px">S/ {{ item.precio }}</span>
</el-option>
</el-select>
<el-input
ref="cantidad1"
type="number"
style="width: 90px"
placeholder="Qty"
v-model="EntradaProducto.Cantidad"
></el-input>
// methods
ChangueSelectProduct(){
this.$refs.cantidad1.select();
this.$refs.cantidad1.focus();
},