uri_separator = '&'
uri_url = 'url=https://my.awesome.url:8090/cgi-bin/project1/qgis_mapserv.fcgi'
uri_username = 'username=user'
uri_password = 'password=pass'
uri_format = 'format=image/png'
uri_layers = 'layers=mylayer'
uri_crs = 'crs=EPSG:25831'

url_with_params = uri_separator.join((uri_url,
                                      uri_username,
                                      uri_password,
                                      uri_format,
                                      uri_layers,
                                      uri_crs))
rlayer = QgsRasterLayer(url_with_params, 'norther', 'wms')
print rlayer.error().message()

<p><b>Raster layer:</b> Provider is not valid (provider: wms, URI: url=https://my.awesome.url:8090/cgi-bin/project1/qgis_mapserv.fcgi&username=user&password=pass&format=image/png&layers=mylayer&crs=EPSG:25831

uri_service = 'SERVICE=WMS'
uri_version = 'VERSION=1.3.0'
uri_request = 'REQUEST=GetMap'

<p><b>WMS provider:</b> Cannot calculate extent<p><b>Raster layer:</b> Provider is not valid (provider: wms, URI: