Edit your .bowerrc file and add the wanted proxy configuration:
{
"proxy":"http://<host>:<port>",
"https-proxy":"http://<host>:<port>"
}
If working behind an authenticated proxy, user and password should be included like this:
{
"proxy":"http://<user>:<password>@<host>:<port>",
"https-proxy":"http://<user>:<password>@<host>:<port>"
}
Usually, the .bowerrc is next to the bower.json. And if there is no .bowerrc file near the bower.json file, you can create one by yourself.