A method for forming a welding spout of a plastic tank from a multilayer parison, during blowing of the parison for manufacturing the tank. The method includes, in a blowing mold including two cavities and an insert, one of the cavities delimiting a recess for forming the welding spout, the recess having a mouth opening opposite the insert: providing a push member on the insert opposite the mouth of the recess of the cavity, capable of assuming a first position set back from the recess and a second position in which the push member occupies a portion of the mouth of the recess; positioning the push member in the first position thereof; blowing the parison to match a shape of the cavities; during creeping of the parison towards the inside of the recess, moving the push member into the second position thereof.